Corners are good, there have been a few different posts about those over many years, 2 planes intersecting seems to boost an evocation.
I literally never evoked on a mountaintop and don’t do it in the street either.
2 old threads, you should be able to DIY for as long as necessary because it’s the symbols that matter, and in an old mass rit E.A. did say people could draw them or even use a stick in sand and the symbols would work for that rit, people with the Universal Circle all seem to geuinely get something from it though:
Sticks tend to have more binders and gunk in but if they’re what you can get, use those, get resin when you can (if you go that route).
This is for initial evocation, later when you get into it more you may find you need specific things.
I am NOT an expert in this course (and obviously just a volunteer forum mod, not soeaking for BALG here) BUT I had to create my own systems from no books, no mentors, I had nothing as a kid so I got used to improvising, and I get results.