Part of the fun ismaking do with what you have. With a bow and arrow or bait casting with a small sinker, or other method, hang some twine, tie it to stronger rope, and haul up a corner of your loop. Keep it as symmetrical s possible, although Ive had some pretty wierd shapes in the air and they all work well.
Let enough sag in the wire to let the trees sway. If it snaps and comes down, do it again with a stronger line or a little more snag. Dont be Hercules when hoisting the wire. When it feels hard to move and its mostly up where you want it, accept it and start operating.
Our club uses horizontal loops as terrific antennas on 160-80-40m, often erected just for FD, the Ohio QSO Party, etc. Ive used them for 20+ years and rarely had one come down. But nothing is really lost if one does. SO no need to sweat the details or seek perfection.
Ive used #14 solid copper on 160-40m loops ( an assortment of 4 sided and 3 sided shapes depending on tree availability) with some stretching but nothing I couldnt live with. The tighter you hoist it the more stretching you get. So dont be a perfectionist. Being a slacker is ok!