Gasket

Gaskets create a seal by squishing a piece of rubbery material between two pieces of rigid material. Examples of this kind of seal can be found in waterproof consumer cameras that are rated to 10ft or so, as opposed to o-ring housings that can be rated to 100ft or more.

The "production" method of making normal flat gaskets with cutouts is to die cut them. An alternative method for those of us without fancy die cutting machines is to use a laser. Silicone rubber is a good material for this and will let you make gaskets relatively easily assuming you have a friend with a laser cutting machine.