Finally got round to making one of these up and printing it out. Took about 25-30min in total, excluding printing time.
Added an O-ring slot like the mag has to help hold it in place, but the tolerances are tight enough that its completely unnecessary. Lucky as I didn't make it quite deep enough in any case.
Works very well.
Looking forward to seeing how accuracy is affected, if at all, vs the mag.
My Red Wolf has utterly shit accuracy out of the mag but is insanely accurate using the single shot tray and rowan single shot adapter.
Of course A.R.M.S. is ducking and diving regarding the issue. Not surprising considering their affiliations.
Anyway, will be nice to see if the Crown shows some benefit to single loading, which before now was too awkward to bother. Honestly i really hope it doesn't.
Pictures below - bad light but good enough to get the idea. Pretty basic really, but always satisfying when they come out fitting perfectly.