You might try higher humidity. It can take several days for
the film to flatten out.

Yes. Right now I am keeping it in a spiral container, wherein
the film remains laterally flat, in the bath with a hot shower
on. I was going to keep it that way for a while (so it absorbs
enough water) and then to start reducing the water flow, step
by step, to let the film dry slowly and uniformly, until I
switch the water off. Then, hopefully, it will be properly
dried and flat...

After drying up the curl returned ((
Will try keeping it in warm humid air for a longer time.

Anton