Anything over about 1/4" thick plywood is going to be heavy at the size of that puzzle. At the same time, thinner material (and I'm thinking of Masonite) is not

Anything over about 1/4" thick plywood is going to be heavy at the size of that puzzle. At the same time, thinner material (and I'm thinking of Masonite) is not going to be very stable — could end up bowing in the center.