The basket model has thicker walls for increased durability, and a slight widening of the basket opening to lessen the chance of the shell bouncing off the basket's frame back into the chamber. Several 3M-sized screw holes along all 4 sides of the collector basket for securing a bag (works best with a mesh/net bag so you can clamp the bag netting under the screw heads). I used basic 3Mx4mm hex screws to secure the bag since I have a bunch lying around, but any 3M screw length less than 7mm wall thickness will work. 

The securing mount is the exact same part that Rage_Lee created, no changes there. It will take two 3Mx20mm screws, with optional slots for 3M nuts to better secure the basket.

The assembly slides it onto the Firefly's picatinny rails, and works best with a sling mount or sight on the one side to prevent the basket mount from sliding around too much. 

Rage_Lee's Original Shell Collector Design -
https://www.thingiverse.com/thing:5165975/files