[Danny]

I've previously used chunky screws and washers to fix certain holds to non t nutted boards, but I'm vaguely aware that a specific adapter exists... can't seem to find anything on Google?

[teestub]

Like these?

https://www.climbingwallservices.com/products/the-lonestar-screw-washer-m5

[steveyo]

Have you tried searching countersunk washers?

[Danny]

Like these?

https://www.climbingwallservices.com/products/the-lonestar-screw-washer-m5

Just the ticket. Look ideal. Not mega cheap though.

[Danny]

Have you tried searching countersunk washers?

Good suggestion. Didn't know these are a thing.

[tk421a]

https://www.accu.co.uk/en/solid-countersunk-washers/24503-HKW-M6-A4
Is what many walls use. (When using M5 screws).