MIMF

We are students at De Lage Waard in The Netherlands. At the moment we are working on a project for our final highschool exam. We want to make a simplified acoustic Lap Steel Guitar.
The idea is to make a rectangular box with strings. The box is made out of a top plate which is 36.4 inch by 15.7 inch and a bottom plate is 35.4 inch by 15.7 inch. The top plate has a hole for the acoustic sounds. The thickness of the plates is 0.1 inch and the box has a thickness of 4.2 inch. At the end of the top plate we want to fix the tuners like a normal electric guitar like the picture. We think that the string tension is too strong and we would like to make a bracing to occur this problem and strength the top plate. Would you like to help us how we could place the bracing the best way? And if you have ideas how to place the tuners, we would be glad to hear it.

We have also added a picture of a paper made model and a drawing with the scale of the plates in centimeters.

Not that I think it will sound very good, but a very simple brace installation would be an X braces between opposing corners with a transverse brace between the X legs directly under the floating bridge, tie your strings off to the end of the box with a simple tailpiece and let the bridge just sit on the top, held down by string tension only. String height over the body should be around 3/4" at the bridge, and meet the body near the top so the string angle over the bridge is around 10 degrees. 36" is OK for overall string length, you may want to aim for a scale length (between the nut near the tuners and the bridge of around 24" - 25", so you can use normal guitar strings. Braces would be 5/16" - 3/8" wide and around 3/4" tall spruce, tapering towards the ends down to maybe 1/4" tall. You could also do a longitudinal brace between the end blocks (where you fasten the tail piece and the headstock where you fix the tuners) floating just under the top. That will directly take up the string tension load and reduce the pull on the top from the strings, but will still let the top vibrate.

The design you are proposing is basically that of a zither, so you might want to research zither design. Measure the length of the strings you want to use and make sure they will work with your dimensions. Zither pins would be a cheap and easy way to add tuners, so you might consider using them. Good luck with the project.