I am quite small in stature and I tried out a Dynarette in a shop once, but decided it was not for me.
My left leg slopes fairly acutely when I sit on practically every chair, where ever I am! So, for a while I used a Gitano device on my guitar as well as a footstool.
But a couple of years ago I converted to the Ergoplay Tappert model. I tried the Troster one, but couldn't get comfortable. The beauty of my Ergoplay, is I've been able to ditch the footstool altogether!
At home, on my own chair, I do find it slips, occasionally, when wearing certain trousers, but I think this may be more to do with faulty posture - I think I may, at times, pull the guitar towards me, with my right arm, so this causes the instrument to move on my left leg.
At other times, when I have no choice of chair, I sometimes find it slips, simply because the chair is too high, so my leg slopes anyway. So, very recently, I've started to tape a bit of rubbery material to the underside of the Ergoplay, where it meets my leg. I think the stuff is carpet, or tray grip material. I got it from a fabric shop. It works a treat!
Maybe you should go to a good guitar shop, if there is one accessible to you, and try some different designs out. I think you should ditch the footstool, completely, if you've got leg pain. I don't know how tall you are, but I managed it.
"We are all in the gutter, but some of us are looking at the stars." Oscar Wilde