Tricky Dicky wrote:

I found out to my cost when trying to install boxes that only
accommodate PB up to 12mm. You can get boxes that will fit thicker PB
and that is probably what you need to specify when ordering.

These claim up to 18mm

https://cpc.farnell.com/pro-elec/19679d/double-dry-lining-box/dp/PL04304?st=dry%20lining%20boxes