> I saw a link to a member's van online and noticed a flue cap--
> Is this a production item available on line or someone's handy work?

There is an official VW part.  It probably came with our campers when
new, but many PO's lost them.  I got my replacement from Bus Depot.
I've read that some margarine tubs will fit it fine.  The stock part
is white.

It's supposed to keep spiders and other critters from going in and
clogging it with webs or nests.  Also to keep out water during heavy
rains or while washing.  I leave mine on all the time except when
trying to fruitlessly light the fridge.