By sewing the long edges together you would get a tarp that is 10'x7' (nominal) if you hung that such the 10' dimension is your tarp ridge line length then you would get decent coverage for moderate rain. However if you are going to go the DIY route, then just pick up a 12'x10' poly tarp from a hardware store/Walmart, cut off the grommets, do a rolled hem along the perimeter and add tie outs. You could even make it a hex tarp if you were inclined. Or you could find some inexpensive 1.1 ripstop nylon and do the DIY silnylon treatment. Lots of options.
I got the impression from the OP that they were looking to purchase an inexpensive tarp.
Cheers
Brian
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