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The big deciding factor for me between pre-fab and home made was the amount of time and effort involved. I consider myself relatively handy but designing and constructing a cab was just not worth it when I could buy one for 70$. I went with a home depot melamine cabinet. (Which you have to buy a back board for because the one they include is cardboard.) You can check out my construction thread Here. Mine is a bit bigger than what you are looking for but maybe you can find something that can help. Make sure to take a lot of pictures when you start building, I'd love to have a look.
Heat is vented into the room where a window is usually cracked. With that size cab, I would recommend against a 600w, I have a bit of trouble with my 400w in a cool-tube (7o above ambient can get rough when it gets warm here.)
Good luck with your cab man, let me know how I can help.