Another suggestion would be to put in a vertical piece of plywood, kind of like a spar. Measure the ID of your tube and cut a 1/8" piece of plywood to fit inside...full length or at least the center 1/2 or so. Glue it in with epoxy and just be sure it stays vertical when you mount the wings.

This is the arrangement on my A-10 wing tube and since the loads are all in the vertical axis, it makes the tube super strong...probably more so than just upping the thickness. Cheaper and quicker too.

Craig