a little warning, the advice that schu777 gave about using a box fan for an exhaust fan is a VERY bad idea. I am sure he meant no harm and was only trying to help , however the electric motor in that box fan will be ventilated which exposes the brushes to the vapors that you are venting. which are highly flamable, and brushes spark and arc, alot. in other words BOOOM!!!! DONT DO THIS!!!
And plastic sheeting gets magneticly charged. that means it will atract alot of dust, otherwise known as overspray. and then when you spray it will get blown off of the walls and float around until it finds something to stick to, usually what your painting. use the canvas painter sheets.
Make two pannels with furnase filters, put them on opposite walls and just let the breeze blow through. this should provide enough ventilation for sprayin a bike frame.