I'm with Bill here. You don't need a pattern. Let me apologize to the OP first for my rant. It is not directed at you personally, just an observation I have seen several times here on the MYOG pages.
There really aren't that many patterns for MYOG out there. If there are, you have to pay, because someone had to spend time making them understandable for others to follow. There are a few dedicated souls out there that do make patterns for everyone to use, and we really appreciate it, but this is a rarity. Also most OEM's who have patterns, don't want others making their stuff.
Most of the people here who make stuff don't use patterns either, because they are making it up as they go along. A good practice for this is (as Bill recommended) to buy cheap $1 a yard fabric and make a mock-up. A tarp and quilt really don't need patterns because of their simplicity. You should be able to just look at pictures of other projects of quilts and tarps and that be enough. Clothing is harder and does require a pattern, but you can find those at a sewing store. And, if I can make a backpack without a pattern, anyone can. I know it seems a daunting task, especially with expensive high performance fabric, but that is why you make one with cheap fabric first and learn for yourself.