I appreciate the ingenuity, but a tarp as a ridge line has to hold the same silhouette as a real slope—no soft folds that look like a sheet of cloth. The canvas weight can add too much drag, making the figures look like they're struggling against wind. I’d test a lighter weave and layer it so the paint stays crisp. Also, remember the color palette has to match the terrain, or the whole scene will look out of place. Keep the folds clean and the texture realistic; that’s the only way it will survive both rain and the eyes of a discerning player.