I’ve finally found these HDD caddy trays for my MD1000, which are very expensive (surprisingly!) from official sources. So I bought the full package, all 13 I was missing
The last, annoying, bit are the interposer cards that plug in to the SATA drives and slide with the tray in the MD1000. These are too ridiculously expensive and even harder to get it seems, but hopefully I’ll have some luck on e-bay once again. I’ll need 13 of these, obviously, for a beautiful 15x1TB of storage (possibly 15x2TB, but these aren’t officially supported by Dell, so I’m not sure I’ll be able to use 2TB drives in the MD1000).