It is my understanding a lot of the IMAC planes have 2 to 2.5 degrees of right thrust built in. I'm currently rebuilding a 60 size CAP 232. Stock right thrust was some where around 1 to 1.5 degrees. It is now 2.5

Build a new box to fit around the outside of the old box. This will give plenty of surface area to glue to and even a mechanical fastener or four would be a bonus. Use the old box as a guide to construct the new box. I'm guessing the box has tabs to lock it together. Triangular stock can be use to reinforce the joints in loo of the tabs and a little fiberglass cloth will add a bit more strength.

I'm sure others will chime in with more ideas.