Light Proofing

Im trying to make a two door steel office cabinet into a grow chamber. What do you people use to seal the corners and cracks between the doors and frames of your grow closets? I want to make a lower shelf with a 24hr light for seedlings/clones and then a 12/12 chamber on a shelf above for flowering. What can I use to keep the light from leaking outside of the cabinet when the lights are on and from leaking back into the cabinet when the lights are off. Here is a generic image of basicly what Im working with.

Sandusky 4-Shelf Storage Cabinet available at OfficeMax.com

Any suggestions or known solutions?

foam tape

yep foam tape, its like 1.95 for a 50 foot roll at the depot.