I'm a drama student who will be doing a partial-nude scene in a couple months (February 1st). Unfortunately, the one part of me that will be showing is the one part of my body I'm not happy with - my butt. It's almost non-existant, and has two large indentations, one on the side of each cheek, near the hip joint.

I'm willing to do pretty much anything to create a more even roundness, but I am limited: I have VERY bad knees, so anything I do must be extremely low impact (squats and stairs are particularly painful). I do have access to a small gym.

Any thoughts/suggestions on exercises I should try, that will give results in two months?

lol, a concave butt

Exercises like glute-ham raises, romanian deadlifts, good mornings, and reverse hyperextensions should round you out. Squats are very good for the gluteus, so maybe you should look into the technique of your squatting before you write it off completely.

Attatch a theroband to your ankle and abudct your thigh out to the side
as well as thigh extensions.

At home lay on your back and raise your pelvis as high as you can focusing on your glutes.