Most people say that tin foil is not great because of the creation of hot spots, due to creases, and also the reflection of heat. Mylar, when properly installed, seems to be a very good option. Flat white paint is probably the easiest. It reflects quite a bit of light, but more importantly... it does not reflect much heat. Depending on what type of paint... it can be cleaned fairly easily. Plus it makes it look like an operating room :rasta:

But hey, Shibby... I was thinking:
What does the back of the speaker look like? It's probably got some speaker wire inputs right? Could you take that out and run a tube from the grow space floor out through the bottom? Not sure if this is making sense :stoned: I'm looking at your pictures and trying to brainstorm. Even a small piece of thin ductwork could be attached to an exhaust fan with a reducer... that would at least move some air out of the box. Take some more pictures of all sides. Even the bottom, underneath. You could put it up on a crate or something and exhaust from the bottom. Just trying to picture places with open spots you won't have to cut.