I did link to a Youtube in this thread which explained a little about extra drives in the T3500.
I belive that without any extra cages/expenses you may put a 3.5" drive hanging on it's screws inside abeam the power button or not far away.
Anyhow I did buy the Dell WH216 drive cage which have a fan for cooling, which fits 2 HDD, just below the DVD.
Then I replaced the DVD with a Startech 5.25" bay which fits a 3.5" HDD and a 2.5" SSD with a included small fan at the rear.
The 6th 3.5" is so far loose inside at the "floor", I will try to fit it more secure soon.
(It would have been better to just get a cage which occupies 2x5.25" and yields 3xHDD).
My last 2 SSD's fits anywhere.
The 6 HDD's are connected to the LSI-card, the 3 SSD's are connected to the internal SATA-ports.
Did need some power splitters. And put another 80mm fan at the lower rear.
Easily connect and hot swap dual 2.5” and 3.5” SATA/SAS hard drives from a 5.25” bay
www.startech.com