Just digested this whole thread in about 10 minutes so I may have skimmed a few. What I get is that the o.p. put a 12W LED fixture over an 18" x 18" x 24" enclosure and was underwhelmed. I should think so!
I'm not sure why that has generated such a volume of opinion. Get a couple more of the fixtures? It's what I would do. Actually, no, that's not what I would do. This is what I would do.