Hmmmmm lots of open ended stuff here.

Get some aircraft ply from your LHS. About 1/8" thick should do. Use a piece of stiff paper to create a template of where the servo tray is going to fit in your plane. Write down the dimensions of your servo's (about 1.6x0.8") decide where they will fit on your template, and make sure the control rods and servo arms won't interfere with each other. Double check everything. Use the template to cut the ply and cut the rectangular holes for the servos. Glue the servo tray into the fuse, add "tri-stock" to reinforce. You will want to use the servo tray positioning to influence the CG of the plane, so try to balance everything at the right spot before you permanently install the tray.... i.e. installing the servo tray should be the very last thing you do in the construction process.

gus