A custom 4-5" filter rack isn't all that uncommon.

I do prefer/recommend the Honeywell F100 though.

This post was edited by ryanhughes on Sat, May 4, 13 at 1:20

The problem with having an open slot in the return is that it will cause a significant amount of air leakage. It is not so bad if the furnace is located in a conditioned part of the house. It means less air will be pulled from the return vents and possibly air noise. But if the furnace is in the attic, then you are pulling in cold or hot air, and dust through the slot.

If you make a slot see if you can also make a door out of sheet metal to seal it.

System is currently in basement (partially finished).

now there is just an open slot and I slide in the filter, obviously not air tight...but doesn't seem to be bad. Of course I know that will get worse the bigger the slot, but figured it was worth a shot....and maybe I could just use some tape or fashion a door of sorts.

...and if I mess up too bad, I can just buy the "cabinet" from AprilAire or Honeywell (mentioned above)

Hhhhmmm, I just thought of something though. My current filter size is 14X20X1. So far most of what I am seeing is 16X or more.

This post was edited by RascalMJ on Sat, May 4, 13 at 16:36