thinking further about this. I have never seen an 8 way grid blank/cover plate and I cant think why anyone would (yourself excepted of course) have need to use one. so I think you might have to make one or change the back box to two doubles/ how many xlrs do you need to mount?
MK used to do a large blank plate that fitted there back box for large cooker switches. It wasn’t perfectly flat though, a little curved. Fixing holes top/bottom, not two per side. It looked shlt TBF.
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