Lay the plexi directly on the LCD frame, but make sure to cut or drill some vent holes in it first. Never tried anythiing like this myself, but if the issue is that heat would get trapped under there, it seems like a logical fix to me. A few holes scattered regularly across it shouldn't interfere with minis and gameplay, but would still allow heat to escape. You'd want several holes so that it would be less likely to have each of them covered by a wrong configuration of pieces blocking all of them.