You may have to take scissors and slice up the pot scrubber to fit that narrow space. Certainly do-able. I would keep an eye on it and make adjustments as needed. The built in overflow at the front should let you know if the water flow is being impeded too much by the pot scrubber.
I've wanted to put ceramic media (because of their high surface area) into HOB filters that have vertical cartridges but could not figure out a practical way to do this until I tried the following:
Take a length of fishing line and thread it through a handful of the "noodle" style ceramic media rings and slowly drop it into the filter using a zig zag pattern leaving a ring or two at the top. This allows you to fill the available space efficiently and makes retrieval of media simple during filter maintenance.
Sent from my iPhone using Aquarium Advice