Summary: use trim board to make your shower more narrow (60") so it fits their largest size.
We ordered the 48". The shower wall was not flat because our shower was the two piece design. The lower part was more narrow. We made up the difference by adding a strip of
PVC trim board from homedepot.
This might likely be what you'll want to do... on the side that holds the roller place the PVC trim. I attached the trim with screws to the wall. Both of the Nautilus shower sides attach with double-sided tape, so just attach it to the pvc trim board.
I was forced to put the trim board on both sides and used some very small vinyl quarter round moulding (zoom into the photo) to make it look like it was factory installed.
You should not need the quarter round because you'll likely put the big side, with the roller, against the wall alongside the cabinet, just like we did, so quarter round moulding would not be visible anyway.