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.

• Flush mount horns: Pic 25 to 31