O-Rings are a common pneumatic component that can easily create an airtight seal between things. They can be found in everything from fire hydrants to pistons. With the OpenSCAD script on this Thingiverse page, you can create 3D printable molds for DIY silicone O-ring gaskets.

Materials:

Filament (PLA or ABS)
EcoFlex 00-30 (or any other low-durometer silicone)

Tools:

Clamp
Scissors
Cup
Spoon
Screwdriver
Small syringe

Step 1: Customizing

First you have to use the Customizer page to set the molds to your parameters.

Parameter "Inner Diameter" defines the inner diameter of the ring

Parameter "Thickness" defines the diameter/thickness of the ring itself

Set the sliders to the proper values and click Create Thing>Create Thing, making sure under the "Part" tab that both molds are selected.

Step 2: Printing the Molds

Both molds should be printed flat with the following settings.

.1-.2mm layer height
10% infill
4 shells

Step 3: Casting the Mold

Put the two parts of the mold together so that the sprues align and clamp them. Mix up the two parts of the silicone thoroughly. Syphon it into the syringe and literally inject into the sprue hole. Let it cure for 4 hours.

Step 4: Demolding

Use a screwdriver to pry open the mold. Peel out the O-ring and snip off the sprue and any flashing with scissors.

Step 5: Use

How you use your new gasket is largely up to you. The flange.stl (also on the Thingiverse page) can be printed to demonstrate an O-Ring with 10mm ID and 5mm thickness and is held together with M3 screws. Since the Customizer page is meant to be more of a tool, you'll probably already have in mind what you are going to use this for.