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Consider strapping (used in construction). I think the dimensions are 1/2 x 3 and sold in 8,10,12,14 lengths. It is typically southern yellow pine and #2 or #3 on the grade scale.

While it has some flex, you may want to get it wet and prop up the middle of the length to get it to bow. Adding cross pieces in the middle of the U will keep it from sagging.

I too use a heavy duty tarp. For the frame I use saw horse brackets and 2x4's for the legs and ridge. One set in the bow and another from the console to the stern.

My boat is parked perpendicular to NE so it gets the nasty winds on it's side. So far it has only collapsed once and since tying down the frame it stays put. 

I have the tarp closed at the ends and very tight (prevents snow load). 

I have recent photos and will try to upload them.