Hmm concrete. Sounds like it would work. I have a friend who wont buy a chiller and got sick of using 2 lt's. He did almost the exact thing your thinking.
He took a large rubber maid, cut the correct holes for his 1/2" barbs and such. He lined the bottom of the rubbermaid w/ insulation. Then he set the normal controller/res (witch was the same rubbermaid just smaller) and hooked up all the tubing. Then he used Great Stuff foam insulation around the cracks between the larger rubbermaid and the controller. He also lined the larger rubber maids lid w/ insulation. It brought his temps DOWN. He didnt give me numbers he just said it worked great. I also had him build a box for his airpumps w/ 4" ducting blowing in cool AC air.

I started thinking a little after this and you could use a larger cooler to place your res in as well. Or just use the cooler as a res. They would all help w/ temps.
I also think concrete would bust most rubber maids but I could be wrong