Then why use foam?

It
does take up space. Be sure to use a method that allows for changing plants/pots in case one dies or gets too big. Also pot drainage.
I have a feeder of similar design. I was surprised to learn I need to take it out frequently for cleaning, but I expect that applies to all feeders.

I have it attached with 2 zipties clipped short—loose enough to remove & replace as necessary for cleaning, but tight enough to hold it without slipping (and support the weight of a cham, should that happen).