search ebay for "3u server chassis" and you'll get tons of options that will hold up to 16 3.5" drives. May need to do some googling on the model numbers offered to confirm if the backplane is included, motherboard form factor compatibility, and so on. Something like this seems like it'd be a good fit, but again do your research first:
https://www.ebay.com/itm/Supermicro-CSE-836BE16-R920B-3U-Server-Chassis-2x-920W-3-5-16Bay-BPN-SAS2-836EL1/283672465787?epid=1100073804&hash=item420c32517b:g:iqQAAOSwDMtdydEa
I'm running a 1950x now ad am happy with it, plenty of cores for everything I've come up with thus far. With Threadripper gen 3 just about to drop prices on these have been going way down.