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How do you plan on fastening the UHMW to the runners? I used 1/4'' x 2 1/2'' aluminum and bolted it to the inside of the runners and then bolted the UHMW to it. If you use a solid hitch I would attach it to the skis and use it for jacking so your not jacking on the frame of the shack. Looks good and you should have no problem hauling it with your sled, just pack a trail and go.
I'm wondering what made you decide to notch? If I were to do that, it would be for added strength because I intended to glue and screw. There would have to be a benefit for giving up 3-1/2" of ground clearance. Otherwise why not just put em on top if you're gonna be using hurricane clips anyway?
I wanted to go for added strength, reduced weight, and not have it be sitting too high off the ice once I block it up. I decided to go with joist hangers and I'll also be using some screws and wood glue just to be overkill.