I finally got around to put the order in for the 21mm Birch plywood (front panel 24mm).
I will be using my Tannoy Berkeley HPD 3828 15" drivers.
They are a copy of plans found on T'internet but to the same dims as Mr Coco's (which I've heard).
Idea is to make the round front panel cut compatible for both 3828's and Golds.
My Coco kindly put their data into his horn calc prog and things came out very much the same as for his 15" Monitor Golds (see below).
Monitor Golds params
Monitor Golds response
Apparently the comb filtering is normal. So green light on HPD's working on paper.
I'll post up some pics as we go...