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Putting one higher up in the middle is an interesting idea. I don't see why that wouldn't work, might be worth a try.
Doesn't two snowboards kinda defeat the purpose, unless they are longer than your sled?
my thinking is the two snowboards are as wide as the sled already, making the same amount of friction...
I really like idea of attaching the roll up sled to the center of my monster smitty for deep snow. Mines too big and heavy to pull by hand. However, mine is designed so that the skis ride in the plowed wheeler tire tracks. My thought is that with the skis riding in the relatively low fridction area of tire tracks and the weight being on the two contact points, the deep snow drag in the center should be less relevant.