What are some good bum exercises aside from Squats and Lunges?

It seems like the main thing everyone recommends, but I can't do them. I had a pretty serious knee injury a while back and I can't do any proper ones with weight.

I still need to work on my butt though. It's small and firm, but kinda flat and not really round like this link link link

And I'm aiming for something like this link link link

I mean I know I'll never look like Beyonce but I'm looking to gain a bit more volume and just roundness.

I do a lot of Pilates exercises like Donkey kicks, leg lifts... I also do the Stair master about 50 min a week and the elliptical about 200 min a week. Have been doing that for years, but I don't see any results yet.

Are there any other good exercises you'd recommend?
What are some good bum exercises aside from Squats and Lunges?
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