I was wanting to get some pics to show you, but didn't have time.
So let me walk you through easy way to design a counterweight before you do something crazy,
First , open and prop the door at the correct position you want it to be, then take a piece of cardboard and cut out an outline that matches the front of the door all the way to the back side of the tank. It should look like a set of bird wings when your done. Now tape the door half to the door. The close the door. Now looking down the length of the cooker, draw a circle on the upper half of the cardboard that represents the back half go the tank as high up on the card board as you can go with out crossing behind an imaginary vertices line coming straight up from the hinge pin. That will be your counterweight. Now you can take any point along the part of the cardboard that is touching the door and use that point for your attachment point. Just draw up what looks good and cut it out and use this for your pattern. The only thing now you have to do is figure out weight. No..it does not have to be the same weight as the door, only enough to give it leverage. Best thing to do is make the two brackets from your stencil you just made, tack the onto the door, then start tack welding some srap onto the top of the brackets until you get the right weight to get the feel you want for the door.
Ok?