If you want to tie your own, take any boat rope, like anchor line or halyard. An 8mm rope has a breaking point of about 1500kg depending on the brand, and a 10m rope has a breaking point of about 2000kg depending on the brand. The tricky part is really fastening this thing so that the attachment holds firmly. But it should be possible to solve with a wire attached to hooks or the like, or perhaps a sturdy beam with lots of holes screwed in place.
I had the same thought about ropes of various kinds; there can be a price difference from much cheaper to much more expensive. Then it's hard to give a good description of the execution, and as you write, the fastening device is difficult when you are not on-site and can see how it is; the conditions are often not the same for all of us.
If I had used my climbing ropes, it would be cheaper to order a ready-made net. If you buy thinner rope, it works fine, but it becomes time-consuming and perhaps not as good as the factory-tied ones. The factory-made ones are often spliced, not knotted, with strands split and braided into the next mesh, and many fall protection nets have a metal core, guessing a stainless wire...
But I would have just for fun tied one myself... Probably gone crazy after a while and ordered a finished one...
