My rotary table need a chuck also. I am going to get a 6 or 8 inch 3 jaw chuck. My mounting plan is....
Get A chunk of material to make the mounting plate, something about 1.5 to 2.0 inches bigger than the chuck and about 1" thick. The material will be whatever I can scrounge up, alloy is not real important.
I will start with the back plate on the lathe and face it and turn the register fit the back of the chuck. Then drill whatever is needed for mounting the chuck to the back plate and mount it semi permanently.
then I will put a chunk of stock in the lathe an turn it so that it is on the exact center of the spindle. Then I can mount the new chuck to that stub and it will have the chuck and back plate on the exact center of the lathe spindle. Now I can face the back plate and turn the register to fit the rotary table and the alignments should be perfect with no accumulating tolerances.
The outside of the back plate will be bigger than the chuck with mounting holes to bolt it to the T slots of the rotary.
Whenever I make fixtures for the rotary I usually put in a pin sticking out the bottom to locate it irrationally the same each time. I will probably do the same for this chuck also.
My lathe is a D1-4 mount so I do not want to eat up enough of my Z axis to make a D1-4 mount for the rotary. So I will not have the option of bouncing back and forth between the mill and the lathe.