I also use a crockpot for my poor man's sous vide. There are a lot of instructions on the web for making your own temperature control relay but why build one for $40 when you can buy one for under $30?
Easiest thing in the world - set the temp, set your deviation (I do 1/2 a degree), plug the crock pot in and let it work. It's amazing how well that thing controls the temp of a crock pot. A large roaster would work as well.
I did a chuck roast at 132 for 36 hours (yup, I know, it's beef
) and it turned out as a can be.
Haven't done any venison yet but this post has put my sous vide bottom round on the top of the menu. Well done!