I use a thin coat of West Epoxy on the balsa skins, and clamp it all together. Works great. I do the work in a spot in my garage on the concrete floor where I have determined the floor is absolutely flat. For the top, I use a piece of 30mm granite from a sink cut-out I saved from a kitchen remodel. This stone is heavy and flat to a very high degree. After placing the granite, I set four 5 gallon full water bottles on top of that. This adds about 330 lbs. to the 75 lbs. or so for the stone.

Sufficient clamping force and flat surfaces are critical for having wings set up without warps!