I'm still interested in alternatives.
Here's one which someone will be along to criticise any moment now, that's what the internet's for after all
I never quite got on with the arm loop, so my spanner lives on my harness. You will see:
* it's in 4mm cord rather than elastic, so it needs to be long enough to reach to the full extent my hand can stretch to
* the potential overlength problem of a big loop round my knee is dealt with by an alpine butterfly mid-cord so I can tidy the full cord loop out of the way when I need to
* the harness gear-loop end of the cord is a double figure eight with the spanner and whole cord fed through it so it can be unlooped and transferred to someone else if necessary but remains secure enough that I can't ordinarily chuck it down the pitch unless I really want to
* the spanner end has a slight overkill extra loop in a different size\colour cord so there's something to catch my eye and I can tell the difference between the different knot loops without having to think at all hard - I don't know whether this is more or less complicated for whatever (lack of) vis you may be operating in, but it works fine in air!
* it all hangs nice and tidy on a small krab when it needs to and can just dangle on the cord if I let go between bolts
It works fine for me. Your mileage may vary!