You probably already know this but Weihrauch do a single shot mag which is excellent for about £35. Not a tray I know but so much easier because when you cock the gun the pellet holder flips out - very much like the Daystate one. The breach area on the 100 is quite small so loading via a tray would be tricky I would have thought.
If you haven't seen one, I have one on my HW100 and go to MAD quite often. May see you there sometime.