ORIGINAL: littlera
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!
Don't mean to rain on your parade but did you check your math? Last time I checked, a gallon of water weighed about 8 lbs. Multiply that by 5 = 40 times 4 bottles equals 160 lbs. That's about half what your weight is unless the bottles each weigh over 8 lbs on top of it.
Concrete blocks are another good and relatively inexpensive weight source and you don't have to worry about spills or leaks anywhere.