>>1985060nta but what exactly are you expecting it to cost? It's a pretty niche thing you're looking for which puts up the price. A crust Clydesdale fork (only cargo fork retrofit I can think of) is $355, and you're looking for more complexity than that. I wouldn't be surprised if you're better off money and time wise just buying a readymade cargo trike.
Also seeing as you never specified location I don't know why you're annoyed about French products.