I couldn't find a waterproof overmitt that I liked, so I made some from a Green Pepper overmitt pattern. They turned out surprisingly well.
I made the first pair exactly like the pattern called for with Ultrex (2 ply) from Seattle Fabrics, and then I made the second pair with what I considered improvements. I stitched elastic in the wrist instead of using the wriststrap on the outside, used Ultrex for the whole mitt and no gripping cordura fabric on the palms, because the cordura wasn't waterproof and didn't really add any gripping strength, and cut the hem of the gauntlet right at the edge and made a rolled hem rather than the deep hem called for. Then I seam-sealed them.
I wear them as overmitts with the possumdown gloves. They're easy to pull on and off for quick moments of needed dexterity. They have kept my hands dry in snow and rain, and seem to add a bit of warmth. When it gets colder than mid-twenties I switch to my primaloft mitts under the overmitts. The price was right - a yard of fabric is about $12 per yard and will make 3 pairs (Mr. B got a pair).
They are easy to sew and a DIY project that's worth the effort.