Adding M3 nuts to a 1515 frame after assembly can be a big hassle requiring disassembly of your carefully-squared frame. This solution allows you to add an M2 nut after assembly, allowing you to add stuff to your printer you didn't plan for (like filters etc). Just slide the nut holder in so that the tabs are in the horizontal part of the channel, then the M2 nut, and you're ready to secure all the things! See photos step1,2,3 below. Tweezers help for this. If what you're attaching is designed to be held by an M3 screw/nut, no worries, just use the m2to3 spacer.

Tested on Makerbeam XL profile. YMMV.

Hope this helps!