I don't lie on the ground much anymore for photography, but when I did, I would carry the aforementioned Glad Garbage bag. I would try to go for a better class of big, dark green garbage bag (Glad if I recall was a favoured brand) for thicker plastic, think it was anyways.
One other thing I did back then and still do today, is carry a smaller white garbage bags (ie; kitchen wastebasket size) carefully folded flat, in my back jean pocket...in case the rain came down in torrents....WR Pentax or not...I would cover camera body and lens as I hotfooted it out, to the safety of my car.
After all, I'm not ashamed to admit, that I'm generally a fair weather photographer.