from your little sketch it looks like your too worried about intake and not worried enough about exhaust.
in my cabinet,which was like 15dx21wx35h, i used my sunleaves 250 pulsar mini. in that light the ballast and reflector are all one unit so i was dealing with more heat than something with a remote ballast. your space is roughly twice as tall as mine was. I use a small squirrel cage fan but i still had temp issues, once my exhaust timer got messed up and the cabinet got up to 112 degrees!
my advice is to spend good money on an exhaust fan and use passive intake. right now i bet your blowing more air into the cabinet than you are removing....