One thing to remember is that when you purchase a large computer case, aka a full tower case most likely there will be 2-3 120mm fans. Computers run Hot and cases are designed with that in mind. The HPS shouldnt be a problem. Honestly, Line the inside of your case with foil and loose the reflector for the light and you should be able to mount it fairly easily in the top of the case.

I really like the idea of your grow and let me know if you get a grow log going