The foams are rectangular and are meant for use with HOB Hagen AquaClear 150s. I buy them in the "three packs" because they are cheaper than buying them singly. Foams are definitely cheaper than buying new media that comes with the Quickfilters.
Anyway, the foams stay inside the cylinder because that sized foam is large enough to hold itself inside the cylinder without being too big. They slide in and out of the cylinder for easy cleaning.
I actually remove the whole powerhead and Quickfilter each weekly water change. I break down the powerhead and clean the pieces with a tank-dedicated toothbrush in outgoing changewater. The foams get cleaned in a small container of changewater. I change the water when it gets dirty, and keep sqeeezing the living heck out of the foams in new changewater until the water is quite clear, though it never seems to get absolutely clear.
If you use any kind of filter media, whether the media that comes with the Quickfilter or the foams, the media will need to be thoroughly cleaned at least once a week to prevent crud build up. Crud build up causes two things - 1. reduced water flow, and 2. accumulated crud that breaks down into nitrate and phosphate leading to nuisance algae problems over the long term.
Also, if you cut off the bottom of the Quickfilter, you will have to use a cable tie around the bottom of the remaining cylinder so it holds the cylindrical shape. The cable tie should be tight enough to stay on, but not so tight that it squishes the cyliner out of shape. Without the cable tie, the foams will fall out and the cylinder will fall apart during use.
HTH
