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

Step 3 Set the curvature if the retaining ring

Wrap plastic around the mold, securing in place with duct tape. Dip the mold into boiling water for 20-30 seconds, this will set the plastic into a coil with the same diameter as the mold.

I've found if I just bend the plastic, and don't use thermosetting, that the lens tends to burst apart after a few days.