re: Filter box for shop mini-split

Pete,
I went low tech with my minisplit filter upgrade.  I started to make a box like Dave did but because it is so close to the ceiling, the filter box was going to have to be on both sides of the unit.  I tested the airflow using cardboard and that configuration restricted the intake even without a filter and significantly reduced airflow.  By simply laying 4" filters on top and sealing around them with duct tape, the air flow only drops by between 5 and 10 percent when I measured outflow with an anemometer.  I also measured amps drawn with just the fan running with and without the filter and there was no measurable increase in amps with the filter in place.  When I have opened it up to inspect the internal filter (just a screen really), there is literally no dust on it and no dust that I can see on the condenser vanes either.  I suspect that my my nearby DIY Corsi-Rosenthal box fan air filter does the heavy lifting as far as keeping the dust out of the air and away from the AC filter (there is very little on the filter even after more than a year) so I suppose that this is really the second line of defense.  I use a motion sensor to turn on the CR air filter so it is always on when I am out there and it stays on for 20 minutes after I leave.

Note that you may need to seal around the front door of the unit as well to prevent it from sucking air around the door and bypassing the filter upgrade because of the increase in resistance that the filter may cause. 

I had to order custom sized filters from Filterbuy.com to get ones narrow enough for my minisplit.  The cost was reasonable.  They did not have an option to order ones that were long enough to completely cover the intake so I ordered two shorter ones and simply taped them together to form one longer filter.  IIRC, I ordered Merv 11 filters and I highly recommend 4" filters.    

EDIT to add: Another thing to consider when adding a box as Dave did is that because the front door pivots up, you have to make provisions for that so that you can inspect and clean out the inside from time to time.  I was going to have to completely remove the plenum box on mine so just removing the tape on the filters is much easier.