Reusable filter for NPET water fountain (WF210+ and possibly other models)

The NPET WF210 water fountain has a filter cage which contains a stack of two elements: a sponge for catching hair and other large contaminants, and a paper filter filled with activated carbon and KDF-55 filter media. The paper filters are meant to be disposable, and eventually start to become impermeable to water, causing the fountain to back up, but this is a design for a reusable filter. While the fountain runs fine with just a sponge in it, if you want to keep your water cleaner longer, activated carbon can help. This design is for a filter cage meant to sit either above or below the sponge, or even replacing it outright, with a removable lid, which can be filled with large activated-carbon granules (frequently sold as aquarium-filter media). You'll want to pack it in tight, or else water will just carve holes in it. Give it a rinse before use to wash off charcoal dust, and make sure there aren't places where the granules have been displaced completely after washing it. If there are, fill those empty spaces and rinse again. I haven't used KDF-55 filter media myself, but I think it has a much smaller granule size and is unsuitable for use in this filter, as it would just wash through the holes in it.