I can’t get the post mount caliper close enough to the hub on my Fox36 forks. I was considering a small (0.5mm max) shim behind the disc centrelock to stop the incessant rubbing (not a lot of friction but enough to make it sing a lot). In my opinion this would still leave more than enough thread engaged on the lockring.

Any thoughts/experiences with similar problems?

IMHO their is plenty of teeth engagement for the centrelock. The teeth will hold it in place anyway the lockring is just to stop it moving away from the hub rather than to retain it on the hub- ie to stop it working you need to sheer all the teeth off it so it spins freely and i don think .5mm is critical to preventing this.

I have never done this
YMMV

I’ve done exactly this on the rear of mrs nosedives bike (required because apparently Orange bikes cant measure things for shit). Seemed ok, she is still alive

Thanks folks

Worked a treat

Bought these shims for future reference, little bit tight getting them over the splines but better that than loose:

https://www.thebearingcompany.co.uk/product.php?id_product=7308