Try this
The most effective way I've ever stretched canvas was to get it wet. (Think wet pair of jeans)
Best to do on a warm sunny day, soak your cover for about 20-30 minutes, wring it out (will take some effort), and stretch the damp cover and snap it in place. It should stretch without much effort if it's warm.
Let it dry in the sun and it will retain it's shape and be easy to snap the next time. If you do this on a day like we had this past weekend, you won't have to worry about creating any moisture/dampness issue in your boat. It will dry very fast.
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