If you have sides, I would weld L angle vertically at the two front corners at the height of just above the hood of the sled. I would weld 2 more back from the front ones a little more than the width of the flip over. Make sure the top of the L angle clears the hood. Weld connecting L angles horizonally to top of each vertical. Now put expanded metal or treated plywood on top and fasten. You could also put treated plywood down the front to act as slush/snow block for your sled. Now you can pull your sled in, then strap your shack on top of the shelf. All the material will be cheap. If you can not weld, a case of beer will get any friends or family in the mood to zap it up.