They sell loading ramps for ATVs, snowmobiles, etc. for trailers and pickups. Maybe one of those could be adapted for your purposes. Note that they’re not smooth sliding – indeed, they’re set up for traction, and you’d need to find something to slide your gear on.
A roll-your-own solution would be a 2x4 or 2x6 with a hook over the upper end. You could cut it in the middle and hinge it (on the bottom of the cut) so it could be folded up. A waxed slider plate might do.