Great advice.......I had considered MDF as my underlay but thought that
would be "too easy". I had strong feelings about the integrity of the
plywood available today. Including Finnish/Russian /Apple plywood.

I investigated the expansion coefficient of the woods I had been considering
and found Maple to be within the limits of the imported plywood. Having said
that, I have experienced some nasty warps with the imported and apple ply.
Makes me doubt the data.

MDF sounds like the best substrate and any backing veneer should have
similar expansion coefficients or the whole sandwich is in jeopardy. My
"homemade" veneers will be at least 1/16" so I should choose my backing
juduciously.

OK, now back to the "sawdust" factory.

Thanks for the advice. Much appreciated.