How about that board they put in showers? Pretty cheap and should be
available at the big box stores.

I just finished building two dingies out of 5.1 mm Luan from the big
box store - $9 a sheet... It is nice stuff and will work like
gangbusters as a headliner backer... Give it a coat on both sides of
urethane varnish... It will out last the boat... If you can find 3mm
or 4mm that's great also...

cheers ... denny - just starting my retirement power cat...