I've done a few large scale marquetry jobs and I used hot sand in place of cauls. I basically made a rectangular frame on the floor out of 4" x 2"s into which I set the pre-sized substrate with the pre-sized marquetry panel taped/pinned to it. I then laid a sheet of a type of plastic film (which I can't remember the name of) over the marquetry and the entire frame.
I welded a large pan together and placed it on a makeshift BBQ of bricks and steel bars. I then heated a pile of damp sand in the pan and when it was well and truly steaming, I shoveled the hot sand on top of the plastic film. It worked perfectly on both occasions.
I welded a large pan together and placed it on a makeshift BBQ of bricks and steel bars. I then heated a pile of damp sand in the pan and when it was well and truly steaming, I shoveled the hot sand on top of the plastic film. It worked perfectly on both occasions.