The thing to remember with these counterweights is balance and proper placement.
Weigh your door to get an idea of what you need for counterweight. If you get the counterweight too heavy or get it mounted in the wrong place, it will pull your door open unless you have a means to latch the door shut.
I think the pic that dutch posted is the most common method for weighting the doors. At one point I thought maybe a guy could use the spring assist springs off of a set of ramps from an equipment trailer. Not sure how long it would work out for since on the smoker they would be compressed all the time, unlike on a trailer where they are relaxed when the ramps are up.
I can't remember who posted it but there was a build on here where the guy used gas assist springs from something like a hatchback car.
figuring this out brings me back to college days and statics class....sum the moments when it is shut and again after you open it a little bit....when shut the moment has to be down towards the front on the grill and after it is open a little, say 6" it needs to reverse directions and be up towards the rear of the smoker.
If you built the smoker then you can make the weights and then just tack them in lace to see how the door works. Too easy move it up and too hard move it back. Now I have to go and preform some brain surgery on our grandson.
I can smoke 350 lbs of pork loins or 32 - 20 - plus lb turkeys at one time. It is currently being used at a local restaurant (P.H.A.T. Daddys) in Marengo, Iowa (Pretty Hot And Tasty). I am currently building 5 more of this size. To sell and one to start competing in some cook offs