Funny. I just read a graphic novel based on this idea last Saturday.
Even if the Soviets were to conquer all of Germany after a failed D-Day, I can't see it happen, though, unless you have both no Free France and a Vichy Regime openly allied with Germany and a fitter USSR and the end of the war. The Soviet Bloc is on the Rhine, and on par with USA strategically (I assume nuclear parity is reached earlier, in 1947 or 1948, to avoid the nuclear strike that would inevitalily follow any Soviet attenpt in Western Europe). I'll indulge in ASB, though.
France being treated as an enemy power, you *could* have a Soviet occupation zone from the Swiss border to a Lille-Rouen line (the shore is out of question, the Brits would space out), with the Seine as the main border (I assume that French colonies have either gaines their independance or under British-US "supervision"). Americans support a liberal market regime (maybe with former, moderate Vichysts), Soviets support their own "national-communist" government, and when France is granted total sovereignity again by the Western Allies, The Soviet-backed governement declares its sovereignity over all of the French Territory, known as the French Democratic Socialist Republic. However, the FDSR is only composed of the former Soviet occupation zone. Although considerable tension arise, war is avoided until Stalin's death, each regime calling for each other's destruction (the Bordeaux goverment and the Saint-Denis government. Saint-Denis is in the suburb of Paris and one of the first communist cities in France. Of course, it would have been called Thorezville after FDSR's "independance"). The Seine and Paris itself are heavily fortified, the FDSR wanting to avoid the departure en masse of refugees to the French Republic. Actually, the only "Paris Wall" would be in the subway network and in the sewers, the Seine itself being the Wall with all bridges either destroyed or guarded.
This, of course, implies that parts of Belgium and the Netherlands are Communist, though, or at least Finlandized. So is Denmark.