The RN2120 and all native OS6 rack-mount NAS appear to be manufactured by Chenbro. See my post at Rack-mount-ReadyNAS-drive-caddy-replacement-spares. Note that I just filled a RN3220 with these: https://www.ebay.com/itm/323985095447 , though that particular listing is for 10. When contacted, he gave me a price for 12, so he may sell you just 4. Most of the Chenbro, including those, aren't set up to accept 2.5" drives, though. Do be careful, as there is a new style of Chenbro server and some much older ones that don't use the same design.
These new Chenbro caddies use a plastic lip to lock rather than metal fingers, so you might want to look at older ones I mention in the linked post. While I have not tried them, the SK33502-10B looks right. Listings on eBay vary wildly in price.
