Most of the roll top bags I've seen use webbing to stiffen the opening and provide a 'spool' to roll the fabric around. Usually the webbing extends beyond the edges of the opeing and buckles are attached so you can either buckle them together to make a handle, or have webbing and buckles extending from the lower corners of the bag so you can turn the bag into a cinch sack. Look at roll-top bags made for boaters. Here's an example:

YMMV. Viewer discretion is advised.