I would think you could go to a toy store and get a cheap kids snowboard and cut it in half and use the front half. That would be a very effective ski. Mounting it would simply be a matter of attaching a block of wood to it with a groove cut for your existing axle to rest in and then a copy of that block flipped to go on top of the axle and screw/bolt it together. Ha, it may not be the most elegant solution but it would work and the ski could tilt upwards and backwards to handle bumps. Good luck, sounds like a lot of fun!
-jonathan