What to use through the tunnel on a beetle/buggy ?

I would think that using flexible rubber fuel line through the tunnel is just asking for trouble down the road. Flexible hose is a routine maintenance item and must be replaced periodically. With the hose being run through the tunnel the tendency would be to forget about it and it would be very difficult to inspect. I'd replace with steel hard line (ideally stainless steel hard line if it were a car I intended to keep). Some people use soft copper tubing. That would be preferable to rubber tubing but I'd probably prefer steel. I believe that most major online VW parts vendors (Wolfsburg West, CIP, Aircooled.net, Mid-American Motorworks, etc.) carry the hardline you need.