Rose Brand has heavy
canvas suitable for groundcloths, other suppliers probably have similar fabrics.
132" Canvas, 10.5 oz., Natural, NFR from Rose Brand
The one linked is 11' wide, so two lengths of it would
cover your desired width, or perhaps three widths will
cover the desired length. Find a
canvas sewing shop (like a tent and awning company) and have them do the center seams. They can simply do a 1" overlap and double needle stitch it, and it'll
lay flat and nearly unnoticeable.
I used to do scenery for a short-run outdoor summer theater, a couple of years we did groundcloths over the platforms to get a smooth surface for painting. I'd
cover the
deck with plastic (rain barrier),
lay out the
canvas somewhat loose, staple down the edges, and paint it with exterior latex. The paint would size the fabric, the exterior aspect of the paint would repel rain, and all was well. I don't know how well this would hold up under repeated foldings, but the new fabric with one heavy
base coat with spatters and glazes didn't degrade much either in use, or from being pulled up and taken back to the shop.
One year, I let the local
canvas shop talk me into a vinyl impregnated
canvas (it was cheap). This didn't work as well - the vinyl coating had a much greater thermal expansion/contraction problem than raw
canvas, and it kept pulling up the staples on cold nights. I finally had to glue and screw 1/4" plywood strips along the ends, partially wrecking the smoothness we were desiring.
Operas often do groundcloths, especially for rep seasons. I've seen them laid out and then just tacked down with small, wide headed nails. As long as there's no wrinkles, trip hazard is minimal. Wagons rolling across a cloth might be a 'wave' issue, though, as they tend to press loose fabric ahead of them.