Project Shellter - Customizable Oxystele sinesis

Why should only hermit crabs wear a shellter?
Now both crabs and people can enjoy Oxystele sinesis "heart" shells!

Create a customized home for your crab, and a stylish necklace or earring for you that reveals where another species lives.

The Customizer is the missing piece to enable a hermit crab caretaker to make the correctly sized shell for a crab. A Sage (www.sagemath.org) version has been in development, but when the Customizer was announced, development shifted to this thing.

This is a first version; other versions will be forthcoming.

Please use and report any issues in the comments.