I too use duck fabric as the top of my "big board" ironing surface. I also prefer a plain or solid neutral color as opposed to a busy multicolored print. A busy print combined with the print of the fabric being ironed just drove me crazy. Too much "busyiness" and multi-color combinations made it hard for me to see to fold and press or to see if the strips or squares were straight. That may be just me but something to think about.
I can't speak to the moisture and heat effects on a base of plywood as I don't use steam in my iron, I just spritz and press/iron. Have had my big board that sits on my regular ironing board for about 10 years and no problems at all with the plywood it is made from.