I believe the 6 and 12 spoke 18" baskets share the same cut out mounting hole size but I havn't confirmed this as of yet, so can someone confirm this. I have 6 spoke baskets to measure off to cut the holes for mounting but I will be putting 12 spoke basket subs in and dont want a sloppy hole and what I mean by this is I dont want any play in it. These are the standard common traditional baskets.
From what I measured it appears to need to be 16 5/8" with the gasket. Thanks, Russ