as fas as alternatives for the slate
alot of things work well like glass,aluminum,copper,acrylic,sliced stone etc
the depth and diameter of the pot,the thickness of the pot walls,the thickness of the friction surface and material
will all determine the sound of the call,also the number of sound ports and positioning of them,internal sound chambers too
box calls are just wood on wood as you put it
however there is more to it than that
the wall thickness,the space between the walls,the length,heigth,shape of the paddle/lid
wood choices matter too.all of these things taken in to proper consideration
and you can make a very fine call that sounds realistic too.
lets see them when you get them finished