Try this, I haven't, but it could work.
-Rip the edges out of these skis and snowboards
-Cut the nose and tail sections off, leaving you with a plank of edgeless base
-Countersink and screw the planks to the top of a wooden box frame, this will flatten out the camber
If you had like 2 snowboards, you could cut out 2 rectangular sections out of each deck, leaving you with 4 perfectly shaped pieces of box topsheet material. Countersink and screw and I think you'd be good to go. You might have to shim the narrower parts of the board profile to get a flat sliding surface though. If you're cutting sections out, this would also eliminate the task of ripping the edges out.
Hope this made sense, it's honestly the best idea I could come up with.