The wood I cut my car out of is, quite disappointingly, very warped, but I had nothing else. So I was quite determined to get it to glue solidly to a flat backer and using 'overkill' it worked.
I sandwiched it between two large flat pieces of wood, clamped it all around and added two 5 litre bottles of water and 4 large pieces of concrete.
The frame, I hope,, will hide a multitude of sins