Jburns, that will work quite well, just please add the spring. Your stem extension matches a Schrader valve and a bike store will have rings in the sizes you have, including the big one for locking. Schraders fit a 8mm hole but are threaded on the inside too. You can get a valve stem that is threaded its entire length, which is good for longer pieces.
I used a Presta valve stem, about half of one, which fits a 6mm hole, has a smooth ID of ~4mm and bigger rings to make them fairly universal for bicycle rims (I think both are about 10mm OD) Here's 2 versions assembled:
Bottom view, excuse that plastic spacer inner ugliness, it is smooth where it needs to be:
And expanded view of both:
The one at the top uses a brass screw and a nut brazed to the cap of a brass tack to stay together, and the bottom one uses the whole tack (It comes stock with little barbs at the top of the shaft even)friction fit into telescoping tubes of which the smallest tube is brazed to a brass fitting from a lighter and is held in place by the fitting's o-ring. My springs of choice are secondary ones in some clicky pens used to stabilize the button. They are nice and light and usually bigger around. Cheers!