It sounds like you’re facing a large scale area, so because of your budget constraints, I would focus on doing something impactful with a large-scale element like fabric or a large roll of paper. You can use these to cut shapes to surround the top, or the front of the stage.
I know that photography stores sell large rolls of paper, about 6-8 feet, for not too much money. You can get a lot of decor bang out of not too many bucks with these. Then use some posterboard to make your shapes or decorations a little more sturdy, if you need.
I would just say to cover as much surface area as possible! Don’t focus on the tiny details or just one corner. If you have leftover budget after the large elements of decor are taken care of, you can have some fun with those!