I've put sealant around the screen, heatshrink on the wiper spindles, replaced the ventflap... Now I just drive barefoot with my trousers rolled up, much easier.

Ooh, please don't use bathroom sealant. It has salts in it that start rust very fast.

I made this mistake once and you should see the mess it made with rust

Automotive sealant is fine and will last a whole lot longer too.

By the way check the wipers spindles also.

....and the wiper spindles?

Chevrons2, those have I already substituted by brand new ones!
Meanwhile I invented a gutter that takes the water to the legs of the passenger nearby;)

Anyway, I can't use sealant because I need the air vent to open from time to time - you know, in my place winter doesn't mean rain all the time and sometimes the air vent is convenient...

In the eighties I had a brand new 2cv which also had this problem and the local Citroën dealer wasn't able to solve it efficiently... I guess this is typical of most 2CVs, isn't it?

I have two 2CV's which are nice and dry in any weather.
I repaired rust under the front window rubber(the rust is not visible without taking the windscreen out), mounted rubber dust caps from brake cylinders over the wiper spindles and mounted a new went flap with new rubber.
No water have entered, in any weather, for 6 years now.

When mounting the went flap, make sure to bend it slightly.(but don't overdo it) When you close it, it is pulled shut in the middle. It is important that it has a curve, so that the ends are pressed against the body when shut. Before mounting, lay it flat on a table or something, if the "outside" is facing up, then the middle part of the flap should not touch the table, but it should be resting on the ends/edges.

This sounds like an excellent idea, I've always thought the majority of my leaks stemmed from the extremities of the vent flap. It's a very long span for a single arm to pull shut tightly. I had wondered about adding another linkage at either end of the vent flap to pull it firmly shut, but decided it was more effort than it was worth.

The other option is to use self adhesive draught excluder on the opening, worked on mine .

i did that too, but it should be cleaned well before. Under the rubber of the flap there was a lot of black dirt that siphoned the water in, after cleaning the rim and the rubber. It was dry. Make shure the draft excluder is made of foam with on closed cell structure otherwise it stays wet and causes rust.

it gets in through the wiper spindles well on mine it did
u need a recon unit if it does- or u can try using grolch beer bottle rubber stopper washers from their beer bottles fitted under the spindle nuts
check if there is any sideplay on the spindles