Hole saw on where you want the rounded end to be then with a cut off wheel cut down and to the side depending on the style and shape you want to go. The new sinister bags are real thin ppo and cut real easy no more plugs and having to fiberglass the the inside of the hole.
Make a cardboard template of the good one, then trace the outline on the other bag. Make sure you use painters tape prior to the outline. Cut with jig saw,hand saw, coping saw or a cutoff wheel. Take your time nd measure twice.