[Closed] Any Cajon players/builders here?

I've made a couple of cajons of different sizes from birch ply.I could do with some advice on maybe altering materials to improve the sound. So far I've made three small ones and one larger. They all sound different (in the bass and the looseness of the rattle of the snare (like the hi hat/percussive sound).

I've used 1/8" birch ply for the skins and this to me seems to be the deciding factor in why they sound different. It's so thin that the grain must affect density and therefore it's ability to move air. Physics is not a strong point of mine so I could be way out.

Any alternatives to birch ply (or different thickness make a difference). It's quite hard to source and expensive from the modelling shop I've bought from.