If you put a hem on the fabric it will roll up lumpy and will never look right. You need something that will stiffen the fabric and prevent the edges from fraying, and not be affected by damp. One way is to dip the fabric in a PVA solution, (diluted till it looks like watery milk), then allow it to dry. You can then trim any loose threads and/or cut to width, and the fabric won't fray. The biggest problem is that the fabric has to be perfectly flat while it's drying. I solved this by clamping one end of the fabric between two pieces of wood, and letting the other end hang free. Always do a test piece with the PVA - you want the fabric to end up neither too firm nor too floppy.