Epoxy works for me. I dont know why it didnt for you. Maybe some skin oils or something like that? As for 200mph hatch covers, I've been thinking on that a bit and I guess you would have to make sure the hatch has good airflow over it. Just speculating but turbulance at high speed might buffet enough to blow it off. On the other hand, with the up wind side of the hatch pegged and a detent on the down wind side in addition to the the magnets, might just hold. There are some pretty strong magnets out there.
Edwin