PolyBox Extension (Centering)

A humble part folks who like both the PolyBox and large (300mm / 12in) diameter spools, such as the 2.3 kg spools from FormFutura, but found that the box is wide enough but a bit short. This challenge has been met before (see links below), but I wanted to add guides to center the spool to keep it from rubbing on the rounds. And speaking of the rounds, I wanted to make sure they were C2.

My printer's 12" square bed can't quite handle the size, so I'm using the split version that can be joined by a pair of 6mm or 1/4" fasteners.

Original model and more on GrabCAD: https://grabcad.com/library/polybox-extension-1

Other PolyBox extenders:
https://www.thingiverse.com/thing:2884426
https://www.thingiverse.com/thing:3156422