Cut 2 squares of canvas 8 or 9 inches square. Sew three sides, fill the bag and sew up the fourth. Use heavy thread like the stuff used to sew leather sheaths and boots. Keep the stitches small and close together, or you can double the seams. After you’ve sewn three sides, turn it inside out to hide the seams and the rough edges. If you use gravel, wash and dry it before filling the bag with it. Only fill the bag half to two-thirds full.
Yeah, I make my bags out of canvas too. Another thing to do is buy one of those cheap grip things they sale at any sports store. When you you squeeze it, make sure you have it upside down. The thumb is always the weak part of your grip, turning the gripper upside down lets you exercise your thumb more.