I use a pyrex 9" square dish almost every time I smoke. I use a loose piece of sheet metal as a baffle in my offset. When it is in place, the top of the sheet metal sits on top of the lip at the top of the opening between firebox and smoke chamber. The bottom of the sheet metal sits on a grate that sits low in the smoke chamber. I use the pyrex about half full of liquid to sit on the grate and keep the baffle in place. This way, the baffle is easily removed, and the pyrex catches a lot of the fat drippings. Whether the liquid is of any benefit in keeping things moist is for another thread.
The point of all this is; I liberally spray the pyrex inside and out with a cooking spray. Not to endorse any particular product, but the one like maP spelled backwards works fine. After the smoke, the stain can mostly be wiped off with a paper towel, but I use a scotchbrite type pad. The stain leaves a little darkness in any nooks or crannies in the glass, but it comes probably about 99% clean with very little effort. I have used that dish at least 100 times with no damage, The baffle keeps it far enough away from the intense heat.
I spray the cooking spray liberally on all interior surfaces of the smoker both before smoking and after cleanup. Nothing ever sticks to the point that a good stiff brush can't remove. No rust, no muss.