Blue Peter version first before I make a plywood version for manufacturer.
I think I'll go for 1¼" stainless steel instead of 1" tubing, given height and its location.
Any thoughts on this mounting method? I'll use existing transom ring bolt and penny washer (as pictured) - thanks DHD for that suggestion. Might get this stainless steel bracket routed slightly so there's a degree of adjustment where it connects to the transom ring bolt.
The lower mounting plate will be bolted through the transom knee, probably 2-3 button bolts. It probably won't sit as flush as I want, but I'll use a bead of sikaflex on the back of the plate, then a bead on the visible edge of the mount for a tidy finish.
Remind me to clean the drain well bilge!