Even easier to do, Emil, but I will just make a 3/4" thick crescent, and not even screw around with the 2" pipe because the arc would not be quite right for a crescent anyway. Already have the jig for that, and for an excluder, and Conley too, for that matter. Only thing I will have to check is whether my router bit will cut 3/4" material, whatever it will cut, I will make it that thick. Don't even need a picture for that, you guys know what that will look like.I do not know if a 2in crescent will allow the martins to enter. If you have a 2in pipe cut to size that the martins will use, and if that pipe is cut to 3/4inch, you will not need sideboards. What I am saying is that a 3/4 inch thick opening does not require sideboards.
You know, if you were to laminate two 1/2" thick Sintra crescents (I don't have any 3/8" Sintra), you could place a mounting flange made out of plastic siding in between the two pieces of crescents, and the space that would be created by adding the mounting flange would provide traction and a universal mount for almost any cavity.