I made a rain/Sun/weather cover for the duffel bag that rides on my rear cargo rack.
Now I'll just leave it on there most of the time. Rather than carrying stuff I'll need everyday, I'll just carry the stuff that I want handy but do not need often.
Rain gear, batteries for my tail lights, stuff like that.
The everyday stuff will go in the two backpacks that mount up front and I'll carry them in and out everyday.
That cheap tarp material isn't all that Sun resistant. But if I Armor-All it regularly then it ought to be as waterproof as I need it to be for, say, a year maybe. That'll be good enough.
The only question left is whether or not I'll get tired of lugging that big old thing around. But I'm lashing the duffel on and off that bike everyday as it is.
And I've always maintained that, for me anyway, two-wheeled transport must have cargo capacity. Otherwise I'll be back driving my car half the time. It means some inconvenience. But it's worth it.