Why Your Lower Back Shouldn't Have to Work This Hard
Whenever the base has to bend their knees, like when entering most poses or when doing pops, that's the point where the lower back comes off the floor. In static acroyoga the lower back should stay grounded, not lift up.
Poses like bird or chair are a good example of this. Putting a triangle or even just a pillow under your lower back gives it the support it needs so it doesn't have to arch to compensate.
It also makes your practice a lot easier overall and lets you progress to a higher level faster, since your back isn't fighting to compensate the whole time.
I personally built the perfect acro triangle for this. It folds up for travel, comes with a belt so it doesn't move around while you train, and it's the exact height and width you need to play comfortably.
Don't have one yet? A pillow or anything soft works the same way, just pop it under your lower back.