I made this mod on my little 3" 3 Jaw chuck before doing this to my 5" 6 Jaw.
The 3 inch runs with less than .001 TIR. The 5" is just a good after the mod.
The key is to have a small amount of clearance on the chuck back plate register at the chuck side.
By carefully drilling a .106 or #36 drill at each jaw and tapping for 6-32 set screws at the very back of the chuck. Just be careful to not put the hole too far back or too forward. It needs to be just right to miss the back of the recess in the chuck and not break out the back of the chuck.
Once that is done you can start to breathe normally again! I re-assemble the chuck to the back plate on the lathe and snug up the set screws. My chuck uses 3 hex bolts to attach to the back plate so I keep them snug and position the chuck with a hardened dowel pin (1") in the chuck and indicate that until true. Keep tightening/tapping and snugging/loosing until it runs as true as possible and your 3 main bolts are tight.
I am sure this is not as good as an Adjust-True Buck Chuck. However it gets the job done for me and it did not cost $500.