I printed the holder in PETG with a high infill ratio. 
There are files for mounting guides. they can be printed to ensure correct spacing and orientation for the holder. Its is VERY important that they are STRAIGHT! otherwise the boxes wont go in. There are guides for ORSY 4.4.1 and 8.4.1.

I found that they are strongest when printed with the outside facing the build plate. though this does mean some support is needed on the inside where the box slots inn. i have not had any issue removing this afterwards.