Well...
I've converted kirbys to a bag system before, but i used a tradition emptor i haven't done it with a square one. If this would interest you would need:
A Tradition Emptor
A Heritage Bag plate (the piece that screws into the emptor and holds the filltube
Fill tube and bag adaptor (i've always went for the round ones)
A Strong adhesive (locktite, Epoxy ect)
I would attach the tube to the plate then bring the plate up through the bottom of the emptor and then seal it to the emptor so that the lip of the emptor covers the edges of the plate the easiest way to keep them together is to hang it up then you just wait for it to try
In this old video you can see what it looks like when finished.
Hope it's of some help
[MEDIA=youtube]1_Aaw882zxA[/MEDIA]