Several ways to make that piece,
I have used rectangle stock centered in a lathe(with most of the middle cut out) then turn a 45 degree bevel into that to help center it on the collar... put a pin in the other end, it gets gripped in the drill chuck.
or drill a piece of round bar, a bit bigger than valve collar on the od, the inner hole at least 100 thou smaller than dia of collar. Then cut the sides out, drill the other end and insert pin to allow holding in drillchuck.
The really cool way is to get a hold of a chopper tech. Ask for used clevis ends off the helicoptors, they look like tuning forks but shorter and are made of Ti. The ones I got (3 different) all had 3/8 fine thread on one end, fits nice in the drill chuck.
If you have a lathe turn a piece of bar to fit in the center hole of the drill table & long enough to reach the valve head without any part of head touching the table.. you could use wood or plastic, or make a piece to fit the dome of the chamber.
The tricky part is holding the head, because 1 hand is holding thedrill press handle, they other is needed to flick out the keepers... so be warned.