That's the plan. There is no need for any significant leverage factor so a screw slot is easy and a good solution. I will be doing up a few of them and send you one unless you simply want to cut the one you have down a total OAL of 95mm (per below info) and cut a slot in it with a hacksaw or small grinder wheel (Dremel) then swipe an air cap from some car somewhere...
It will have to be plastic, because it will be crossthreading, valve stems aren't M8-1.25 and there is nothing out there in that thread that I found readily available. Let me know
I have the inside engine distance at 51mm plus the body/flange nut/air cap stack up at 34mm plus 10mm adjustment range equalling a total of 95mm. That is pretty much exactly the length of a 90mm hex bolt including the head thickness, which is what I will be using.
Thanks for the pictures and input so far.
In a quick afterthought, I may try dipping a bolt in some vinyl tool handle goop, let it set up then unscrew the resulting "cap" and screw it on the slotted bolt. Might work okay and is worth a try.