I used some 1/2"-5/8" green foam and a tube of flexible foam adhesive and primed the plywood middle. I would use much thicker foam next time. I used a few small hand and knee boards from 1/8" masonite paneling. It has supported a pals 280ish without damage ...I have one heel of my palm make a low spot, Palm Pond IMO, structure foam is plenty good enough a surface for most trains when noise and layout weight is also a factor.
Scraps are rocky scenery