What you are describing is common on giant scale planes. You have a couple options, you can build and engine box or use standoffs. The engine box can be made of 1/4" ply for the sides and 3/8"-1/2" for the firewall. I would use 1/4" for the back and through bolt it with 10-32 bolts and blind nuts. I would then re-enforce the area where the the box meets the original firewall with 1/2" triangle balsa on both sides and the bottom. Cut some lightening holes into the sides, bottom and top of the box. I like to coat the inside of the box with thinned epoxy and glass cloth. The outside can be covered with carbon fiber cloth if you like or just thinned epoxy. You may also want to use some 3/16" dowl to pin the firewall to the box sides. The result will be a lightweight and very rigid engine box. Your other option is to use standoffs. This are usually made of aluminum and some even have the thrust angle machined in. Check out http://www.aerografixs.com/ they have a nice install using standoffs.