Here are the pictures of my thermostat housing with the holes in it. I only had one hole but after I start poking and prodding I found (made, whatever) three more. A mechanic type buddy of mine says I should be able to patch them up with JB weld. I'm going to try it tonight just so I have something to throw on there tomorrow.
However, I would love to take a look at your plastic housing. Where did you get it? I have yet to see any plastic ones. I need the one with out the sensor nut on it.