Desktop vents will not be sufficient for a cab with a 400 lamp. You will need to get decent ventilation, best more than 400m3/h - and I strongly suggest a carbon filter.

I am running a 410m3 vent with a 500m3 carbon filkter, a 400W lamp, all in a 3'x2'x6' cab - and I am having difficulties with the temperature. Meaning I will have to increase the passive intake....