I've just come across these: https://www.thingiverse.com/thing:3220573
and: https://www.thingiverse.com/thing:2810089.

I'll print one of the digital versions off, and see if it can be set up to use this: https://www.ebay.co.uk/itm/Black-Car-Digital-Tyre-Tread-Gauge-Depth-LCD-Display-0-25mm-for-Multiple-Purpose/283676395108?hash=item420c6e4664:g:L7YAAOSwR-9dzmr1.

Worth a try, might need a slightly heavier spring for our spokes.