The cases in this thread are cases people like, use or considered when setting up their current or next UnRaid server. There is no requirement that they must hold 20 plus drives.
Few cases come with adaptors, unless you look at Norco custom designed disk array style cases.
3-2,4-3, 5-3 style adaptors are available from several companies.
With your current situation, back to basics.
Do all drives identify at "power on" reliably? no look at power supply. PSU splitters if you have then should be thoroughly checked (power down the system). Pull on each side of the splitter the cable on the other side should move the same and consistent amount if it doesnt it is making a bad contact. This can often be fixed by squeezing the sockets gently so they mate more firmly.
Are the drives recording smart errors? UDMA_CRC_Error_Count numbers usually indicate a cabling error. However these could be from a previous problem so ensure they are still being recorded for the drive in question.
Cable lengths. Try to keep all cables as short as possible.
If a cable is suspect, cut it up. They are cheap to buy, dont leave it laying around. Chances are you will re-use it and re-introduce errors. This applies to backplanes as well. Are all the drives giving issue furthest from the data plug? Try moving them closer to the data socket.
Post up a sys log and we'll take a look and offer advice.