Homemade drivebay

There was this old server case that i manage to pickup, and I want to add some internal drivebays in an area that is completely empty.

a) Should I make it 2.5", 3.5" or 5.25" in general for internal? NM.
b) If I cannot find existing drivebays, what kind of metal-base material can I use for quicker assembly of the drivebays (what I mean is can i use those shelf standards, and if so what dimension and spacing.