You could also ask in the French forum for more timely responses than might come from the US.
How do you keep the collar on the cat? Cats are notorious for getting out of anything.
Many cats are reluctant to push thru a flap. Raccoons on the other hand are not! I'd go with a simple door in a set of tracks (or on rails) that can simply be pulled up and then lowered once the cat has made its mind up to go thru.
Maybe an RFID 'tag' on the cat with a scanner to pick it up once the cat got close enough.