A few weeks ago I made this back plate from a 6" diameter x 1" thick A36 structural steel disk for my 5" 3 jaw chuck. I used my 8-14 bench top lathe and cheap carbide insert tool bits from Bangood..........
Well, I decided to also make a back plate for my 5" 6 jaw chuck because the "registry boss" wasn't/isn't compatible with the 5" 3 jaw chuck. This time however I bought a 6" dia x 1" thick cast aluminum disk for making the back plate instead of steel because machining a steel disk was rather slow going with my bench top lathe and the "stringy steel razor blade swarf" was scary sharp to mess with........
Here is the aluminum disk I ordered for making the lathe chuck back plate...........
https://www.rotometals.com/aep-b-6-aluminum-disc-6-diameter-x-1-thick/