As I stated elsewhere the numbers shown are often ‘nominal’. This includes the individual board width .. I went with 12″ – 0.250″
Thickness : I show 0.042″. In O scale this is 2″ thick. Is this thick enough? Perhaps not. If you wanted to make it 3″ thick then change that to 0.063″ .. if 4″ then it would change to 0.083″. Changing the thickness will of course affect the heights of the walls and some of the timbers. That is why I keep using the term ‘nominal’.
Width : I show 3.688″. This is simply it ‘came to’ adding other dimensions – the width of the four frames and the three ore chutes. This means it can be wider .. but no narrower.
Height : Simply what is needed to bridge between the front and back walls. If the thickness is changed for the bin floor itself or the front and back walls this will obviously change.