My mount and scope stay outside most of the time, only bringing it in when I'm leaving town for the day.
It's covered in layers. a towel being the first, to absorb any condensation, then a quilt for insulation, a blue tarp as a second defence against rain, then topped off with a Telegizmos cover.
In your instance, I'd be putting on a towel, then slipping a sleeping bag over top, then add your plastic bag.
The ultimate pier cover though.....and I want one so bad I can taste it....is this.
http://blogs.discovermagazine.com/ba...on-the-inside/