$5 Macro Lens - for use above and below the waves

Step 5 Make the retaining ring

Fit the coil around the lens port. Trim it such that it is exactly the right diameter, and will tightly grip the lens port when glued shut.

Now, peel off the protective plastic coating and remove the ring from your camera. Use instant glue to temporarily hold the two ends together. When dry, test the fit. This bond isn't too strong, so if the ring fits the camera too loosely you can just break this seal, trim, and re-glue.