Would you consider making one out of masonite or something that, when finished nicely, would slip between the rails of flex and hold it at the curvature you need? It may be a very quick solution that would cost you a few dollars, no shipping, and you'd have it inside of 20 minutes or so.
I actually made a template out of thick cardstock which worked out quite well.
Just a thought....
-Crandell
can someone please come to my aid ie ribbonrail 22" rad. track laying gauge everywhere i have tried is out of stock. any one with a sorce would be onderful