There are two main ways to seal the top of a well, a well cap and a well seal. If your well has a well seal please go to Well Seal for further information and options.
Well caps come in many designs but one that are relatively flat or slightly domed are the best since our included grommet seals great with this sort of cap. If your cap is weird, just replace it with a "normal" one. They are available online, in local hardware or plumbing supply stores. Know the outer diameter (OD) of your well casing before you shop to make sure you get the correct cap.
A well seal
A one piece well cap
A two piece well cap