Back in the mid-1970s while visiting International Violin Company in Baltimore, the owner talked me in to trying some "marine" grade plywood he had on hand for a dulcimer. It was mahogany on the outer layers. It worked well for a teardrop dulcimer. I did not to try doing an hourglass bend with it. I did not weigh it, but I thought it was a little heavier than my solid rosewood dulcimers of the same pattern. I also made a courting dulcimer using the plywood for the back and sides. It seemed to work well.
"The dulcimer sings a sweet song."