I found it easy to source thick walled steel tube (seamless) from M-machine. A quick check shows up a choice of the sizes you would require to produce turned 'T' rings.
I recently produced a pair with a diameter of 4 1/2", although mine had all the spokes riveted to the inner face of the 'T', but easy enough to alternate. Maybe a little fiddly on the 3" Diddy ones, but do-able.
I used a simple double edged (3/16ths) square HSS tool in a boring bar on my lowest speed (ML7) with no issues-

There are some images in my album which may assist you with building them this way, or help you decide.
Rgds