Over on the AC a while back someone posted the period recipe for the covering. Started w/ 22oz Cotton Duck and painted it w/ a mixture of IIRC linseed oil, pitch and rubber but what the exact mix is I don't know.
Both Upper Mississippi Merchantile and James Country offer a nice groundcloth, or used to. I don't have access to either here but it might be worth looking up. $40 +/- IIRC.