I've made a number of lensboards by laminating 1/4" Masonite, using two squares of slightly different sizes for an "instant rabbet". None has ever shown signs of failure, and they were all assembled using Weldwood resin glue waterproof, resorcinol-formaldehyde). This glue was, according to legend, developed originally for assembling Supermarine Spitfires during WWII. The fact that it is still in production (aircraft, boats, furniture, etc.) must say something about its utility. It is mixed with water to a thick-cream consistency, and cures by chemical reaction, not cooling or drying. The desired glue thickness is not large, and nearly all of the water is consumed in the curing reaction, so differential swelling of the wood is not a big issue.

My lensboard assemblies never warped, possibly because I either clamped them securely or used heavy weights. For a large area, I would probably just go and pick up several bags of sand from the local building supply, and use something very flat and stiff (like 1-1/8" decking plywood) to support the work.