That's a good idea.

I wear a rain jacket and the rest of me gets wet. Works all the way down to freezing temps, but I need more layers below ~45 F in the rain. Fenders of course.

I use a bona-fide dry bag to keep important things dry, or I pack my things in a garbage bag inside of may backpack (which goes in a rear basket). But if I had to take a laptop I would get a hard case, the reason being vibrations.