Nice work
I've been trying to figure out a way to build a paint booth for a few weeks for a low cost. I've been trying to research the most efficient and safe motor for the project and everyone says, go with a squirrel cage blower (like the link you provided or a cheaper one at 148 cfm http://www.globalindustrial.com/gcs/prod/30127642/i/productInfo.web?utm_source=google_pr&utm_medium=cpc&utm_campaign=Fans-Agricultura-google_pr&infoParam.campaignId=T9F), a fan using a pulley motor (which there is none for a smaller scale paint booth), or brushless. Well, I went with 4 120mm PC brushless case fans at 75 cfm a piece for 7 dollars a piece. I have a 90 cfm case fan that isn't brushless, but I can feel the movement of the air at 10 feet, so I was hoping 4 brushless ones will suffice.
I haven't gotten them yet, should be here any day. But after having read your statement about the loss of flow when using a duct, I'm wondering what I am to do with these case fans Although, I still feel that I could make them work and actually push fumes out, there is obviously a better solution and I suppose it would be the squirrel cage blower.
As for the placement of the fans/plenum, it's my belief that if you primarily work by airbrushing objects in the direction of the back panel, a cross-draft set up is best suited. If your working on small objects on the bottom surface of the booth, than obviously, the direction of the paint will be forced down before up or forward, so...a down draft would be best suited in my opinion. But this is my personal opinion. Spray can painting would probably be a different story since it comes out in a wider mist pattern, unlike airbrushing If you had a powerful fan pulling 300 plus LFM, I suppose it wouldn't matter where you actually placed them.
I'm thinking that I will go ahead and get the blower in the link that I posted. 148 cfm is powerful enough for a mini table top booth. I don't think that placing the blower on the left or right side of the plenum would be any less affective than being in the center of the plenum. So, avoiding a 90 degree angle should be no problem if you made the plenum wide enough to accept the 3" inlet of the blower.
I tried it with the 90 cfm case fan on temporary cardboard booths. The fan was placed on the short side of a shoebox. An opening was cut out on the makeshift hood, and the shoebox plenum was placed on the back. I ran an incense test and the dead zone in the makeshift hood was barely towards the opening of the booth. I then ran the fan dead center of the plenum shoebox and the dead zone was more so along the sides of the hood, 3 inches before the center/side of the hood but still equally as strong in the center like the other setup. The dead zones are negligible when most of my over spray will be directed towards the center.
Anyhow, before I get too caught up in trying to build a cost efficient spray booth, I has to out weigh the cost that I have already put into it and most likely will lose, with all this experimenting....it appears that I've almost spent the amount that it would have cost me to buy an already made booth vs buying supplies and building my own. Could'a, should'a, just bought this...... http://www.pacepaintbooths.com/pace/