Digging into the old Model Railway Constructors again, I find this fascinating tool. Designed for making point work from component parts, it's quite simply a piece of wood of suitable width with slots in it so the wood can bend. Thus you have a handy track gauge for the curved bits.
Obviously getting a bit of wood the correct width is crucial. Someone who is handy with a plane ought to be able to help, or better still find a man with a thicknesing machine in his woodworking workshop and he will be able to do the job with great accuracy, albeit on a very thick lump that will then have to be cut into thin strips. Perhaps this is an opportunity for an enterprising trader to make these and sell them ? Not sure how long manufacture would take if you batch built but I bet not that long.
For making your own track using PCB or rivets in ply sleepers, this would be fantastic. It just shows, not all the good ideas are new ones.