If you are just gluing a flat sheet of carbon fibre to the honeycomb rather than laying it up I would have thought you would lose a lot of strength, the box section they do is all unidirectional cloth which means all the strength is in one direction which could give problems. Like you say the only way to know for sure is to try it lol.

If you are going down the MDF route then try to make sure the components you buy can be migrated to the next build, especially expensive stuff like the electronics. That is what I have been told by some of the forum members.