Resonator guitar without coverplate.

interesting question. this is purely anecdotal, but I recall reading some years ago that the Fender resos are not as sturdily constructed as the more expensive National/Dobro. so you may have some reason to be concerned, as the cover may indeed be important to the body rigidity. of course, it will also affect the tone, but I'm guessing you will be working mainly on the fitment and action of your bridge. since you are going to the trouble to make a new spider bridge, maybe you can also make a (couple of) bent steel or wooden brace(s) to take the place of the cover temporarily while you are doing your work. this would probably not help you critically judge the sound, but would allow you access to the bridge assembly, keeping the body rigid when under full string tension.

I can tell you that its a weak guitar. Its a little like some of the guitars with a tremolo bridge.
If you loosen one string, the others will get out of tune.
A little missunderstanding (English is not my language), im not making a hole new spiderbridge, just the wooden piece.
In fact there is two of them. Im going to make them out of brass. I got this idea that as the brass is harder than wood it should deliver more "vibrations" to the cone. The wood should "steel" some of the vibes because its softer.
I want to make the guitar louder, its to soft now. Maby im nuts? If brass would be better, why dont they make them out of brass? Does anyone have knowledge about this?

For almost any acoustic guitar, there is a trade-off between loudness (at the attack) and sustain. a brass bridge/saddle may actually make your reso quieter instead of louder, as the extra mass will soften the attack and store a little more energy over time, which increases sustain. maybe. it's difficult to predict before one tries. a very light, very hard bridge/saddle material will give you a louder, sharper attack. if I were choosing woods for this, ebony would be the one. metals? maybe aluminum.

from what I've read about that particular reso, to improve it:

1. reinforce the body
2. replace the cone
3. increase the string gauge.

(1) and (2) probably could be done in either order. don't do (3) unless you've reinforced the body.

If you loosen one string, the others will get out of tune.
That's quite true of any resonator guitar. There's a lot of springiness in the cone(s).
Reso bodies are made out of wood, steel, brass and nickel silver, and some custom builders probably use other things. They all have somewhat different tonal characteristics, and it's largely a matter of individual preference. If you want to get maximum volume and the full tone out of a reso, you need to learn to play with fingerpicks. Picking with bare fingers will always sound muted and dull by comparison. I'm not saying "bare meat" playing doesn't have a place or can't sound good, but if you really want to open up the reso, get some picks!

Oh, and all of the above discussion has missed THE most important part of setting up a spider bridge reso! It's the tension screw in the center of the bridge. The preload downforce put on the bridge against the cone by that screw sets the compliance, or how far the cone moves with a given change in string pressure as the strings vibrate. If the screw is looser, the cone vibrates more, puts out more volume, and particularly has better bass response. If it's tight, the sound gets more trebly, the bass goes down, and the overall volume is less. If the screw's too loose, the cone will buzz and rattle. If it's too tight, you may strip the screw, the bridge may bend or break, or the "feet" of the bridge may dent the cone. The playing feel and string height will also change as you adjust the screw. When the screw's loose, the cone is most compliant, so the string tension will push the bridge down and the strings will be lower. The strings will feel noticeably "mushy" and will move around with pressure vertically or laterally. When the screw is tight, the strings feel tight and solid. Usually the guitar will sound best when the tension is just enough to keep the cone from buzzing. Different people have different preferences, though, and I used to like mine adjusted very tight when I was trying to play with bare fingers and liked the brighter tone I got with a tight screw.

Now to adjust it. You don't have to remove the strings. You do have to figure out whether your screw uses a little Allen wrench, a Philips screwdriver, or a tiny straight blade screwdriver like you find in eyeglass repair kits. Engage the driver in the screw head and carefully start loosening. Count the movement in 1/8 turn increments, so you'll know how to get back to where you started. At some point, if you're careful, you'll feel a change in effort as the screw loosens to the point that it's no longer applying pressure between the bridge and cone. Now, start very gently tightening. When you first feel that slight resistance that means it's beginning to press the bridge and cone together, count from there. Usually the range between 1/4 and 1 full turn tighter than that will contain the "sweet spot." For me, now, it's at the low end of that.

Thanks for all answers!
I have tried the adjustment screw, but i could not hear any bigger difference in sound.
How much sound can i expect from this Fender i got ? I didnt realize it was soft until i brought it to a party with most of my playing friends. My guitar drowned in the sound from the other regular acoustic steelstringed guitars :cry: .
Im gonna fix the bridge rough and than let the guitardoctor in my hometown take a look at it and do the last fine work on it.
By the answers i got, it doesnt seem that brass is a perfect material. "It should be a hard and light material". :idea: Ive been wondering if carbon fibers (i hope thats the right name of it in English) would do the job.

The usual preferred saddle material is ebony capped maple. Some folks have used synthetic materials, like Tusq.

Resos are very directional. You never hear them well when you're playing them. (Unless you're playing one lap style.) Have you sat out front while someone else played your guitar? Sitting playing into a corner will reflect sound back to you, too.