I absolutely recommend the iStar 5 in 3 trayless system in a 4u case. You can place 2 of these, for a total of 10 trayless 3.5" HDD hot swap bays. this

And a nice 4U case like this one this one

I have several of these from iStarUSA. They are great.

https://www.amazon.com/dp/B009XA24Q6?ref=ppx_pop_mob_ap_share