The cabinet is and old dresser I gutted and painted flat white. Its standing on end and is approximately 5 ft tall and snuggly fits 2 buckets in the bottom. I've got a 400w mh hps I'm going to hang from the top from a couple eye hooks I had laying around. So heat is somewhat of a concern.I'm not sure where I should install fans. I have a plethora of old PC fans at my disposal. Would it make more sense to exhaust out of the top or side? Also how do you keep light from blasting out of the fan holes?
Other more experience growers will be along with more help. I believe if heat is a concern, venting out the top is the best idea (heat rises). As for as light is concerned, maybe some dark material before and/or after the fan would help. Might also be possible to use some dryer duct to reduce the light so it's not shooting straight out.
Is that MH/HPS in a cool tube or hood of some kind? Mine is in a tube and I exhaust out the top with a 450 CFM inline. Regardless, I would vent out the top if possible. I've never attempted to use PC fans to cool MH/HPS. I've used plenty of PC fans for my CFL boxes. You've got nothing to lose in trying it. But I would have a suitable backup plan in place just in case the case fans don't cool it well enough.
PC fans won't properly vent a 400W, so you're much better off spending the money on an inline fan. Venting out of the top, sides, or back of the cab doesn't matter as long as it is near the top of the grow space. When you exhaust this way you will be using ductwork so there won't be any light coming out of the holes.
If Heat is an issue Use a cool tube on the reflector will cut heat by 60% or so. As for your venting problem make a light trap for your fans. http://www.thctalk.com/gallery/data/500/medium/intake_light_traps.jpg very crude but this concept works really well. As for where to place u were on the button always vent out from the top and pull air in from the bottom. not only dose heat rise but this also allows the air to move threw the plants.