Larger o-ring of correct thickness
Razor Blade
Super Glue

Measure the Oring, 1 of several ways:

-If old one is available and not stretched or shrunken, use it.
-If old one not available, cut open the new one and lay it in place mark with pen.
-Use string to detemine proper length

Cut Rubber material, perfectly straight on both sides.

Use a few drops of superglue, rub together and hold in place for a couple minutes.