What about a stiff screen supported by rocks here or there underneath your substrate? There is landscaping screening with small and big holes, your choice. OR- You could use that stiff canvas plastic (for needlework) and cut it to the size you want. The problem might be that it would be too stiff to keep your shape. Depends on how heavy your substrate is to hold it in place.
I know that people use clear plastic (like the stuff that is used in retail packaging. It keeps the roots on the proper side but it you planted your ground cover on both sides I would think that would work well.
I know that people use clear plastic (like the stuff that is used in retail packaging. It keeps the roots on the proper side but it you planted your ground cover on both sides I would think that would work well.