Doesn't seem to me that the CG will be effected in any major way because the engine itself is still in the same location as the engine on the plans (though of course probably a bit heavier since they use an OS 40 FS). Obviously there's the weight of the mount itself. But the location of the CG shouldn't change.

Randy, I'll have to look harder at the Puppeteer instructions/drawings to see how the box idea might be an option. I'll also think about the idea of wooden beams -- though as far as I know there aren't any of the usual hardwoods available here in "model sizes." There is one dark, dense wood with a Japanese name I haven't been able to translate yet that might work for beams.