Most 250w system have the transformer in the hood and that is where most of the heat comes from. Your 400 watt system is should be a remote system and that will help just because the transformer is outside the grow area.

Bake-A-rounds work great there are some good threads on building them. I have 4 of them I have built and used over the years. Now they are on the shelf but I still have them. These tubes are prety slick I must say.