I was thinking of getting some stainless tube and making my own.
These on Ebay
are stainless. Don't know how good but at that price, easy to replace, even each season!
They have sizes up to 89mm. Admittedly they are in China, but worth a punt?
Then again, wonder if these would be better
FLADEN METAL BOAT ROD HOLDER GAME SEA FISHING UPTIDE TACKLE RAIL MOUNT ROD TUBE | eBay
or this looks better, but in USA
Rail Mount 304 Stainless Steel Rod Holder with Plastic Liner for Boats | eBay
Looking better still, with plastic insert
Vertical Rail Mount 304 Stainless Steel Rod Holder with Plastic Liner for Boats | eBay
And, just to be silly, how's about a gold plated one?
Boat, Marine, NEW! Stainless Steel Gold Plated Adjustable Rod Holder | eBay
Just shows how the Yanks get all the best gear. Postage makes these things damned expensive. Anyone got a RIB big enough to get across the pond and bring a containeer of goodies back??