Side Lever PE4 + Equipment

I'm just got to side lever PE4, and realized that I can't progress without some equipment. Looks like they're suggesting stall bars and some kind of strap, which is an expensive combo. 

Any other options?

A box and something to stick your feet under works well. So does having a training partner hold your legs on an elevated surface.

I get the box, (I was thinking of using a sturdy book case) but what kind of thing would be elevated enough to stick your feet under if you were up on the box/book case?

I've used the top of a dumbbell rack and a strap a tied between two trees. Sometimes the tops of play structures work too, depending on the set up.