Light or expected rain, I just cover the seat with a shower cap as well and throw my light and odometer in my bag.

I also filled in the allen nuts with wax to prevent water from filling in them.

If it's raining hard, my employer lets me park it in a closed entrance. Otherwise, I would probably get a cheap cover if I had to leave it exposed in the rain year round. I know it's not going to melt, but I feel better and that's what counts...