I think CS doesn't make it because it's not something that's needed very often.
I've cooked for years, outside in the rain and have little/no rain impact the vent hole (or even the controls).
If you think it's a good idea, I'd email them or call them, as they don't routinely monitor the forum for suggestions/improvements, they'd rather hear from owners directly.
For winter, the cover won't help much because the issue will be every time you open the door you'll lose a LOT of heat.
But I say go for it if you feel the need.