The easiest way to solve the knee problem is teathering. My clone knees are tethered to my thighs
[ATTACH=full]112718[/ATTACH] (The padding has long since been upgraded to actual DIY Knee Padding)
The strap - a peice of webbing materiel - is attached going right to the edge of the thigh - inside of course! - and then to the inside of the knee part way down. The very top of the knee sits over the bottom of the thigh, and because of the tether can not drop down under the thigh. Hope that helps
You might also want to attach the biceps to the shoulderbells to stop them from dropping as well.