I once built a very good spray booth and then threw it out. Extraction fans are a great way to pull dusty air from the room across your freshly painted model. Waste of time. Since then I always spray outside in a very light breeze, and get excellent results.
I know what you mean about the dust.
I overcome that issue by using my shed compressor for the air brushing work. I have 2 outlets at the compressor...one set to around 25psi and the other one set to maximum pressure (about 100psi). The low pressure outlet has a moisture trap at the compressor PLUS another in-line moisture trap between the main 3/8" supply hose and the thin air brush hose.
Before I start spraying I use the 100psi hose with a dust gun attached and spend some time blowing all the dust from the surrounding area and through the extraction fan.
My booth is a very agricultural affair, made from 4 pieces of MDF for the sides/top/bottom. The back "wall" comprises of a 16" automotive cooling fan (Davies Craig), speed-controlled by a device I bought off ebay and powered via a pair of N70ZZ automotive batteries. I don't have any ducting, but do my spraying in my shed which is 40' x 25' (also doubles as my workshop and train room). Being a 4 meter high Colorbond shed it is not even close to be sealed and the spray fumes don't come back to haunt me.LOL
well be some dust still circulating in the shed which may "land" on my model/s....but if there is it is not detectable as far as I'm concerned.