Keymod covers and handstop

hartk1213's didn't fit on my keymod rail, so I modified his design so it would. I also made it slightly more ergonomic by rounding corners and making the handstop smoother. The keymod hole snaps should fit without any modification, and they're slightly springy so they'll stay in place. On my rail they're very tight, not coming off by themselves.

The handstop is slightly different in the picture, I made it fit in the same amount of space as one of the normal covers for the model.