I have made charcloth in a tin chocolate box that is 3 inches deep by 12 inches by 6 inches it works but the quality is not the same at all levels I found that the middle pieces needed to go longer. I have also done it in a one gallon round can with a pice of aluminum foil held in place with a rock with similar results the bottom really charred and the middle not so much.
The cloth I have used is blue jean material, cotton undershirt, and cotton flannel.
Making charcloth works best when done in small batches - and in smaller containers.
Plus your pieces of cotton or linen cloth need to be loosely stacked in your container. The tighter they are packed in, the harder it is to "char" all the way through without over-charring the outside pieces.
And Altoids tin, or Sucrets tin, seems to work very well. They are around 3 3/4 inches long by 2 3/8 inches wide and 3/4 inches deep. That same tin also makes a very handy container for your fire starting set.
So use a smaller container to make smaller batches of charcloth, and only loosely pack your pieces of fabric in your container when you go to char it.
Just my humble thoughts to share. Take them as such.
Mikey - yee ol' grumpy blacksmith out in the Hinterlands