My brother runs Vintage Wiring Harness here in Melbourne. He makes reproduction looms for vintage and classic vehicles.
His name is Paul Vermont www.vinwire.com.au He may already have patterns, or alternatively, will make one from a sample loom. He has a number of 203 looms listed in his price list so my guess is he can help.
They are terribly simple but got slightly more complex with two tail lights and the take off points for the non-standard flashing indicators. The buyer would need to specify if he wanted the semaphore function retained. There was no standard wiring pattern for the blinkers on a 203, all fitted subsequently by individual garages.
A bloke in the ACT used to make them up. I think he had a board he used for this purpose, the loom in my car came from him. PM if you cannot work out who it is, its a while ago not sure he still makes them.
Save some money, do it yourself. Very simple set up. All you need is orignal wiring diagram (can provide if you need) and trailer wire. Speaking from experience, as well as others. Good luck!