Sorry, I meant to do this the other day, but here is an explanation with pictures of what I was trying to get across:
This is the light I purchased from a hardware store:
Here is the modified base that I had my dad make for me. He cut off the spike part and put in screw holes for mounting:
You would need someone who can weld to do this as the spike is steel.
But, barring that, I think it would be relatively simple to take a piece of plywood, maybe 1/2" thick or maybe a little thinner, screw a hole through it big enough for the bolt on the light fixture and attach it to the roof of your enclosure or use a screw eye that's small enough that the head of the bolt can't go through it. I think the wood piece would give you a little more flexibility in directing the fixture though as the teeth on the stem of the fixture would probably bite into the wood a little allowing you to mount it at an angle.
Here is a picture of where and how the bolt attaches: