I went to the scrap yard looking for one but couldn't find one the right shape. So I made one out of 2 air conditioning dryers. I cut and welded them together and welded a bung at the bottom to expand and draw from. I used the cap off of something and made and welded a top that the cap fits on. Total cost....0$ works like a charm!
It's on the bottom left in this picture, attaches to the rad with a length of hose..
It has to work better than having a empty section in the rad?