An update: I purchased 3 of the new style drive trays from OWC Europe (costing me quite a bit though: around EUR 94/US$ 106 with shipping/import duties), but at least I don't have to worry about the drives falling off
They were indeed a little "off" lengthwise compared to the original Apple trays, but I haven't experienced any "bulging" towards the computer's side panel. I've so far only mounted two new trays, but hope I won't experience the problems others have had in this thread.
Regarding the drive I decided to dig really deep into my pockets and do an upgrade that would last, so I bought a
Seagate Exos X10 10 TB/7200 RPM/256 MB (
ST10000NM0016) which is an "enterprise" type drive filled with helium (!). Apparently that should improve on performance, I don't know, but price and featurewise it appeared to be a good buy compared to others. It plugged in and worked right out of the box, and with the new trays the new style screw layout was no problem.
I'm currently awaiting a second drive (for Time Machine backups): a
Toshiba 12 TB/7200 RPM/256 MB enterprise drive (
MG07ACA12TE) and hope it'll work out of the box as well. That should give me enough storage space for a while!
UPDATE 2: I received the mentioned Toshiba 12TB drive yesterday and that too worked without any issues whatsoever (once I put it in a "new style" OWC drive tray because of the different mounting screw arrangement -which I believe is now standard among larger HDDs). Just plug it in, format it and off you go!
It's now been busy backing up my entire machine overnight and hasn't reported any issues.