Try
this online store. Scroll down to the sixth box on the page. 1 1/2" deep, with Loomex cable clamps; made out of steel. Made to work in a
green 2 x 2 wall (i.e. 1 1/2" x 1 1/2"). If needed (because your lumber has shrunk to 1 3/8"?) can you not furr out the wall finish by 1/8" to bring it flush to the face of the device box(es)?
This is how I have dealt with this issue in the past. No room for any boxfill (whatsoever), but since you're running low-voltage, this should not be a problem in your case.
Cheers, Ron