I make mine similar to the one by Mirage on this page from Jeff's site. Only difference is that I don't put elastic around the bottom. I put fabric loops along the border at each end and loosely lace up the ends with shock cord. Pretty easy to use, and the same design works well for a weather cover.
Edit: mine's actually more like Vick's, just after Mirage's on that page.
You can get 3/4" velcro in 1yd strips at Jo-Anns in Bangor for $0.99 ea (comes with both sides of the velcro) - might even find it cheaper at Wal-Mart. Grab four of those and you'll have all you need.
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