Make sure when you tap the mount, you use the propper drill size for the tap first. I have often seen people drill the mount with the wrong bit, then tap it and the screws don't stay in tight. It is something you will do a lot of over the years. Get a good bit and tap to match for doing mounts. Also the same goes for the wings. You can use blind nuts or T nuts if you want. 6 of one, half a dozen of the other. I usually just drill and tap the ply.

Good luck