Thank you.
It is plastic canvas, the clear, flexible stuff. After reading other peoples advice and methods on here, most people seem to put an un-stitched piece of plastic canvas on the back of their stitching and then fix the magnet to that. But the magnets I happen to have at the moment are small enough and strong enough to go between the 2 pieces of plastic. They are about 8mm diameter and maybe 1mm thick and super strong! I then sew the 2 pieces of plastic together using a running stitch like an outline around the work. You have to go round twice to fill in all the stitches, I then just push the trimmed tail end of the thread between the 2 pieces along an edge.
It is easier to see on the ones I made in the swap for Lord Libidan.
Stitched pieces and a stack of magnets:

Finished:
front

back
The black outline of the running stitch makes these stand out a bit more I think