I knew I'd seen something cheaper:
£14 from SJS.
Should be able to fit that under the QR skewer, or the axle bolt.
You'd then be able to fit a stopped derailleur, or a Rohloff tensioner to take up the slack when you're on the small ring.
Fit a front derailleur and double chainset, might need a cable stop for the front gear outer, maybe a double cable guide for under the BB?

£14 from SJS.
Should be able to fit that under the QR skewer, or the axle bolt.
You'd then be able to fit a stopped derailleur, or a Rohloff tensioner to take up the slack when you're on the small ring.

Fit a front derailleur and double chainset, might need a cable stop for the front gear outer, maybe a double cable guide for under the BB?