re: Filter box for shop mini-split

Pete, for my first prototype of my approach and before I found that you could get custom sizes from Filter Buy, I cut a 20x20 (or was it 24x24?) 4" filter in half and taped it back together lengthwise using some cardboard  to replace the sides as a proof of concept.  It actually worked pretty well until I finally ordered the custom sized ones.  I think that the filter I used for that was just one you can find at HD or Lowes, though it was probably just a MERV 8 filter.  I was hesitant to put a MERV 13 filter over the minisplit intake for fear that it might create more resistance than the unit's fan can handle.  Since it basically just uses a screen as a filter, my thinking is that it could strain the motor and/or lower the cooling and heating efficiency of the unit.  Even big standard AC units can be restricted if you go all the way up to MERV 13 filters if they aren't designed for them.