I built a couple of motor boxes. On a RadioCraft designed for a big gasser it was still 1/4 in ply, but corner braced with 1/2 in hard triangle stock all around. Same with the box to the fuse, but the sides were cut into the firewall too. Nosen Citabria uses one, never had any problems with that with a pound and a half of engine and reduction drive.

I think if you do your woodworking nicely, 1/4 in with triangle stock works fine. I through bolted the Citabria and it wasn't all that difficult to do with threaded rod stock and lock nuts. Makes it a little more solid and doesn't cost much.