I don't use Paterson, but I do load in a large changing bag.
But it has a box in it!!!
I got a box at Staples, that is flat when purchased, and you 'put it together' and is suppose to be used for storing office stuff in.
Cut out the bottom, ah.. about two to three inches from the four bottom corners.. with that facing the zippers. The open end towards the sleeves.
What works nice, is the MF paper, ect, can be gotten out of the way, into that zone between what was the box bottom, and now is the back, and the zippers of the changing bag. Gets it out of the way.
Hopes this helps with the changing bag, if it is fact, part of the problem.