There are sponges sold by pet stores made to go right over the intake of a powerhead. They'll work just fine going over the intake of a HOB filter as well.
I don't know how 'pillow filler' is sold, whether it's loose fill or layered. I buy 'medium batting' which is blanket filler, like for a comforter, that comes in rolls but lays out flat. That's real easy to work with, cut out a section you need and wrap it around whatever you want to cover. I also use it in place of the flat cut-to-fit filter pads to go in the trays on my Marineland Tidepool 2 sump filter.
The stuff does wear out fast though. In the sump i double-layer it and replace it every week. On my fry/grow-out tank's HOB filter i replace it every 2-3 days.