You are using a micro engine by Giant Scale standards. Please don't overdo the construction. Example - my 1/3 Scale 96" Laser has a ZDZ 80 (80 cc) with a 26" prop on the front end bolted to an engine box with a 50% cut-out (big hole in the middle) 3/8" firewall. The sides are 1/8" light ply supported with a light ply doubler with big holes cut out of it. The bottom in 1/4" light ply with four big lightening holes cut out of it as well. The box has no top. Works great!

For a 130 FS (21 cc) I think you can easily get away with a 1/4 birch firewall corner braced to 1/4" light ply sides and bottom. The top can stay open. You could even add some lightening holes on the sides.

Light planes fly better than heavy ones!