suggestions to build buttock muscle

I have posted before that I have sitting pain - it is apparently most likely to be some irritation over tendinous attachments at ischial tuberosity. Nothing shows on MRI. The physio I have seen told me to use leg press, leg extension and curls on the machines at the local gym. My hamstrings are weak but the main thing is to build glutes in particular to try to 'support' the area.
Physio set me up to do sets at 80% max of one rep. I am very exercise compliant but unfortunately I am getting increasing twinging pain around kneecaps when on leg press machine. I have dropped the weights down but know that will now be much less effective - I believe it is fast twitch muscle I am supposed to be targeting. Its three weeks till I go back to physio - are there any exercises I can do that will build buttock muscle with less stress on knees?
Many thanks

Single leg bridges:
http://www.tinajuanfitness.info/exer...g%20bridge.htm
These can also be done on a swiss ball! and made quite alot harder as you have to control the rotation of the ball.

Clam Shell:
http://imagine123.com/fitness/tag/clamshell-exercise/

There are 2 simple ones that won't stress your knees too much.

Have fun.