Banjo brothers makes some nice ones that are waterproof: Seat Bags | Banjo Brothers. Pretty reasonable prices too. I'm using this one (Waterproof Saddle Trunk | Banjo Brothers), which can expand to be enormous but also squashes down to a pretty compact size. I like the expandability because in winter it will be nice to be able to stuff an extra layer in there. I just got it recently, so I haven't used it in winter yet, but I did try it out stuffing all my flat fix stuff, some snacks, a fleece jacket, and my cell phone.