The only thing Walker provides is a small bag with a shower cap in it. That is designed to go over the control box to protect the electronics. From a technical standpoint, that is the only part that needs protection from water.
I will use a poncho which will cover me. It will go over the back of the seat and in front will at least go to over my knees. Since I usually wear shorts and sandles, I don't worry about my lower legs or feet getting wet.
I will normally not park it outside if it rains, so I don't really worry about getting the seat wet.