I just use a standard scuba dry bag, which is splash proof.

The 100L version of this bag might work, if you are really tall. The smaller ones will work for smaller legs. I personally use the 40L version of this bag. But you would have to find another version since it appears to be discontinued.

Though since I've gotten a waterleg, my walking leg spends most of the time in my car. And I use the waterleg to get me to the water.