Xavier, you seem to be complicating things more than you need to. Strapping or bungeeing damp rain gear to top of rack works well, and if the sun comes out, they dry pretty quickly.
I'll repeat, what has always worked for me is keeping my rain gear in a front pannier with my cooking kit, so putting damp rain gear into a plastic bag in this pannier isn't a big deal.
And strapping stuff onto the rear rack dries fairly quickly. Open then up and hang on the bike in the sun when you take a break, et voila.
Bonne chance avec ton dileme.