Thanks Brendan. Some of what you said makes sense. 20 years as a structural engineer does give me some understanding of forces and stresses
I'm not going with the glued approach. Decided to use bolts. The focuser baseplate will bolt through the OTA and also through the stiffening plate. Then I'll have some other bolts at the ends of the stiffening plate as well. Haven't done careful analysis or anything like that. And I'm not chasing composite action. I reckon the plate alone will stiffen things up enough, so it doesn't really bother me if there's no interaction between the OTA tube and the plate, although there will be some interaction there anyway.
I measured it all up last night and will be sending off the specs to a local metalworker today. Plate will be 3mm thick, 150mm wide, 300mm long (about a third of the tube circumference), and have a 60mm diameter hole in it for the focuser. I'll drill and tap the holes myself... ok, Peter will do it, but I'll be watching.