what surface would liquid wax adhere to best?
I think that you need a surface with very low wettability, because as you said in the comments, you want to remove the solidified wax layer afterwards. It is possible to make a very thin layer of wax by spreading it between two hard plates pressed against each other. I would try something like this:

You spread the molten wax between the two sheets made from an appropriate material placed between two plates (glass or metal). To precisely regulate the thickness of the wax layer you use some separators placed near the edges. After setting you remove the plates, then carefully remove the sheets. The main problem is to choose a suitable material for the sheets. You can experiment with various materials. I think that a sheet made from, or covered with a silicone rubber would be perfect. You can try a parchment paper, aluminum foil, glassine and so on. If you need a really thin and fragile wax layer, you can try to use a tin or aluminum foil, and then remove it chemically using diluted hydrochloric acid.
EDIT: Now it hit me. There is another, simpler method you may try. If you want a wax plate with a uniform thickness and smooth surfaces use the procedure above. However, if you don't have those requirements, then simply pour the molten wax on the surface of hot water in a large pot. The wax should spread evenly on the whole surface. Leave it to cool down and collect the wax plate. It may be a bit thicker near the edges and its surface will be matte, but maybe it'll suit your needs.