Gotta be honest..a 70w MH/HPS isn't going to put out that much heat. If you could just leave your door open with any sort of basic fan operating while the light is on during flowering, you'd probably be fine. I definitely wouldn't worry about a cooltube, which is great for the bigger lights/grows like GaGrown is talking about, but probably isn't worth it for a small area.

Andā??again this is just my opinionā??you're talking about an area where you'd be better off with CFLs. Because you get better canopy penetration and more efficiency, HPS is better for large areas. But you're talking about .04 of a sq. m. I would consider making your cab a bit largerā??maybe 30cm x 30cm. A 70w HPS puts out about 5400 lumens, so it's enough for an area of that size. You'd be in the ballpark with 3 26w CFLs as well...and would probably eliminate cooling issues. Just my .02.