I wanted to mount the Reprack spool holders directly on my Montgomery shelves so I remixed the cleat to fit.

I used M5 heated inserts on the back side of the mount and M5 nuts to secure the cleat to the shelve sides. The hole will fit an M6 if you wanted to use countersunk bolts through the front and a nut on the back.

Note - you must flex the bottom protrusion on the bracket section when sliding the cleats down. I printed in PETG so it's flexible enough to get it mounted. Would not recommend using this if your brackets are printed in PLA or any other more rigid materials.