How about this:

http://www.bhphotovideo.com/bnh/cont...ughType=search

Can you just plywood over your current holes and use one of those darkroom louvres? They really work great, totally lightproof, and if you install it on the side, they take up no precious floor space. Ignore the part of the description where it says those are designed for particular fans or whatever; that doesn't matter. They work fine for passive intake. I'm using two of them on my flowering cabinet, and I'm quite happy with them, no regrets on those at all.

I read somewhere that going from 99 percent lightproof to 100 percent lightproof can result in 30 percent better yield. I don't actually know if that's true, but the mere fact that it *might* be true is enough to motivate me to go to pretty insane lengths to keep light out. But as long as you keep most light out, I think you'll be fine, however you decide to do it.