Roland, are you asking how to recess mount the horn flare into the baffle. If so, one way to do it is to use templates. Here are the comments for each photo:25) Screw horn down on first Template board.
27) With router base against horn, route out Template-1.
28) Use Template-1 to create Template-2. Router base against Template-1.
30 & 31) Use Template-2 to route out recess on front baffle. You will need to use a template guide on your router base. The size of the guide will depend on the size of the bit(s) you used for Template-1, Template-2 and final recess. Take your time to do the template-math to figure out the size of the template guide. Then do the math again several times to make sure you come to the same answer.
How did I figure this out? Simple. I asked.
http://www.audioroundtable.com/Craftsmen/messages/311.html
BTW, if you flush mount the horn by routing a recess into the baffle, you may run into the same problem of not having enough material to screw in the horns.
Gar.