Back by popular demand is my instructions on how to build a spray booth for around $40.00 (pricing is approximate, your actual cost may vary)

Approximate costs:

Storage tub:$10, dryer vent kit: $10, AC fan: $20.

Materials & tools:

Plastic storage tub (as large or small as your typical project), clothes dryer vent kit (typically 4"), 4" AC powered fan (preferably brushless), air conditioner filter foam, AC cord (built in switch if possible), cardboard, screws (machine or wood, or nut/bolt sets), pen, utility knife, screwdriver.

Photos are of the completed project, but construction is fairly straight forward.

Up-end your tub so you're looking into it horizontally. Mark a 4 inch circle wherever is convenient for your work area layout. Cut the hole with the utility knife. Place the fan over the hole inside the tub and mark the locations for the mounting screws. Likewise, take the dryer fitting of the vent kit and mark the location of its mounting holes. If you're lucky, both sets of holes will be the same.

Route the AC cord through the hole (or make a separate hole) and attach it to the fan. Cut a piece of air conditioner filter foam and mount it over the fan. Depending on the style fan you have, you may have to make a cardboard spacer to make sure it clears the movement of the fan blade. I hadn't attached the foam as of this pic.

Make a cardboard insert a little smaller than the size of your open window. Mark and cut out a 4 inch circle on it. Attach the house fitting from the vent kit to cardboard. Attach the hose to both fittings - window insert and spray booth.

I keep the cover of the booth handy. After I spray a part, I put the cover on the tub. That keeps any remaining overspray inside the tub and speeds the drying and venting processes a bit. When the tub gets too coated with paint, just replace it.