Well, we've got A/C, however it's not warm enough to run a unit without freezing the condenser. I live on the top floor of my building, though, so we get everyone else's heat, and for some reason everyone here really really likes their apartment super hot, even in the spring. Plus, the A/C is a room unit that's in the living room, not my bedroom. Money can be saved if the heat is just vented straight from the light to the outside. That way you don't have to cool that hot air, you can just dump it straight outside.

As far as the plex, I'm basically in the same situation. I can't dump more money into the system right now, so plex is cheap, and super easy to cut with a table saw. If it were to burn, it wouldn't be good for it to fall on the plants, but the products are harmless, just water and carbon dioxide. And apparently, the melting point is reasonably high, about 140 degrees celsius.

Karmaxul, as far as the insulating, I'm somewhat skeptical about how well that actually works. The reason is that when the hood and light reach static thermodynamic equilibrium with the surrounding air, the heat flux out of the system into the air will be constant. For insulation to work, the bulb would have to be a constant temperature heat source. But isn't a bulb more likely to be a constant heat flux source? To be a constant temperature heat source, it would need to have a temperature sensor built in to adjust the current going to the bulb.