I had the problem with my heels lifting up and over the heel support when I first made the moving footplate
I cured this by replacing the heel support with a bit of angle ally (about 2" high)
Have you thought about raising the plastic footplate as a whole
You can buy box section ally on e-bay so if you get couple of bits the same width and length as the ally support undr the footplate you could bolt them to the support and them bolt the footplate on top
Jason