Hi Tadportly
A very hard question to answer with all honesty. I've been reading where the Germans used in China a number of FPW artillery peices including War trophies from the French that were of a larger calibre and antiquated at the same time. Checkout Wargames Foundry FPW range.
Carlos from TMP mentions using the Cannon Fodder Krupp gun and remodelling a barrel to increase the calibre. Adding a new shield can provide a number of variants.
I was looking at the London War room for the 12pdr Naval Gun carriage, a new barrel from the same company looking very close to a 4.1 inch German gun and large wheels from the Brigade Games Gaslight range to make a Konigsberg gun. Something like that anyway
Its not really good tidings actually :|