Unless there is one specifically for your machine, I doubt you'll easily find one that works out of the box.

You should probably be looking at reusable desiccant cartridges. They are a self contain box that absorbs the moisture and then change color when saturated. You can then swap them out with a fresh one, and dry out the used one for later use again. You can dry them in a microwave in a couple minutes.

The foam grommet idea is probably your best option. Get a sealable tupperware container from a home center, and mount it to the top frame. Cut a slot in it to have the post (sans arm) slide into. Bolt the container to the frame, using washers with rubber gaskets if you're really concerned. Then seal up the slot opening with hot glue. This will make the box removable if needed, the bolts come undone, and the glue can be pried off without harming the frame. Drill a hole for the filament to come out of, and 'plug' it with foam as you mentioned. Throw a couple reusable packs into the box, install the arm and mount the roll. Seal up the box. The clear (hopefully) box will let you see the condition of your filament and the desiccant as needed.

You can go with a powered dehumidifier if you choose, but the power usage, draining solutions, and just another thing to fail, make it overly complicated in my opinion.