If you are considering making your own tarp/shelter (you should) you might find one of these little gadgets a great help. You can buy a set of three (universal snap-on fit) from eg eBay for around $10. I will certainly be buying one. It should save me a lot of pinning. My finger arthritis also makes such delicate work difficult.
What they do is double roll the hem to a defined width so your stitching is neat and uniform. I imagine you could also use them (with double thickness) at a join to instantly create a quasi flat felled seam – anyway one which would remain substantially waterproof if seam sealed on the outside!
However, you can also buy a flat felled seam foot and a double needle attachment (at least for my Janome Excel 18W machine – good introductory level machine) which should do this for you neatly and automatically.
You will still need to reinforce tie-out points, and to catenary cut the sides so that the tarp erects in a pleasing taut manner. However, another eBay search will show you that you can also buy a flat felled seam foot quite cheaply too (less than $20)! Even if you have to buy the sewing machine, the saving on just one project will pretty much pay for it. Learn to sew, it will be good for you.
Here is a video showing how to use the hemmer foot:
PS: As I said in this post, you should be able to make a tent/tarp which weighs around 200 grams for from about $20!