Thanks for the replies guys.

I tried putting a couple of holes in my narrower end apron, 4in, to start with, and found that it didn’t hold well after a few uses and became completely useless pretty quickly. I think contamination on the outside of the holdfasts transferred to the inside of the holes, so I’ve cleaned them off, but it’s left those two holes unusable.

Before drilling any more holes I’m going to do a few more thorough experiments in scraps than I did to start with, just in case the hole size/depth is a problem too.

Matt