Hi, want to incase some shells with water clear polyester but don't know what to use for a mould. I only need to see 1 side and polish it as i want to place it in some cement. Help please!

If you need the cast to be a specific shape then you can make your own mould with silicone rubber.

Or if it doesn't matter what shape the cast is then you can just use a mixing container like a plastic cup or take away container. Just do a test to make sure your resin doesn't attack the plastic, most will be fine but for example the typical red party cups will melt in about 30 seconds with polyester resin.