I can gve you 3 options to secure the knee more securely to the PJ.
a/'Calf-Cuff' cost about £80 I think. Officially not available in the UK at the moment. You have to know someone on the continent to buy them for you and get them to send them to you ( a couple of friends have done this).
Basically, its a replacement for the knee bar. A wrap around padded pad about 4-5'' wide.
b/ Go get a couple of those roof rack webbing straps and wrap them around your legs and tie tight.
c/ 'Pinhead 69' straps ( google you should find them). Basically a refinement of the above. Its an 'X' shaped arrangement of wbbing with quick release buckles and a strap to secure the arrangement to the knee bar. Cost is about a £10 inc p&p.
If you look at the picture below, the 4 people on the left. 3 have option b whilst me ( the 1 waving
) have option c