I don't think I've seen one anywhere but if you can't find one it would be fairly simple to make up from some plywood
filler. That way you could make it any shape or size you wanted.
Assuming you have the materials, it would take an hour or less to get the ply cut on a table saw and frame it. Then figure an hour for the first coat of epoxy
and filler to round out the inside corners. Then a second hour to sand the inside and apply a second coat of epoxy or two part paint
. Let it harden and she's ready to install. You could even pre-drill the holes and seal them with epoxy before the installation
, it would never rot