Nice, you won't regret it. Yours is much prettier than mine (as usual). My research found that the minimum filter you want is Merv 11 (see chart below). I went with Merv 13 because it was sort of the sweet spot between cost and efficiency. Going up to Merv 13 from 11 boosts the efficiency of 1.0-3.0 μm particles from 65-79% to over 90% and the 0.3-1.0 μm particles from under 20% up to 75%. I also went with 4" filters to improve the air throughput and reduce how often I have to replace them. My measurements of throughput indicate that my setup can filter the entire volume of my shop about 3 times in 15 minutes at the fan's highest setting. At 90% efficiency, that will theoretically get you 99.9% after 3 times through the filter. Likewise, getting up to 75% on 1 μm and below will get you to 98.5% after 3 times through the filter.
Another thing to note is that some fans can actually suck air from the front and sides of the blades which reduces the volume of air that it will actually suck or push through the filters. Using a cheap anemometer from Amazon, I verified this effect. As I moved the anemometer from the center to the edge (outflow side), the direction of the airflow actually reversed. The round shape of yours may prevent that but looking at the grating and how it curves back around the tips of the blades, I would not be surprised if it pulls a little bit of air in from the front around the sides of the fan. You may actually increase the air flow through the filter by making a cowling that covers the curved edges of the grate so that it cannot pull in air from the tips of the blades. An anemometer can help you determine if that is necessary. To see if you actually increase the air flowing through the filter, use the anemometer to measure at the filter surface how much air is flowing through it with and without the cowling.
--Nathan, TX. Hire the lazy man. He may not do as much work but that's because he will find a better way.
