I have 2 Hunter air cleaners in my home. In the last few years, good quality carbon pre-filters
have been hard to come by. The majority of them now sold in stores/online are very thin.
Even see-thru !! My tower air cleaners are different sizes. Cutting the pre-filter to fit is easy.
In my experience, the 3M Filtrete brand is the best one out there. Nice and thick.
I used to be able to buy them locally, but not so anymore. I'm not sure - maybe they have been discontinued.
K. Fox