George,

So the ideal surface would be a rigid flat surface (thin ply?) that is
suspended?

Yes, but it doesn't necessarily have to be plywood. You could probably use
MDF or maybe even Masonite. I know that RPG makes membrane traps using very
thick/dense vinyl. But plywood is the only material I've used, so that's all
I can vouch for. And thanks, Nick, for reporting your experience with
paneling, because I'm often asked about that too.

I saw on your web site that you don't make the panel-type traps
anymore...I guess I'm wondering why

Yes, we discontinued those about a year ago. They were expensive to build,
and too large and heavy to ship by UPS or FedEx. Once we developed
MiniTraps, which work better than panel traps in almost every way, there was
no need to continue the panel trap line.

--Ethan