For rubber to leather contact cement is the way to go. Clean both surfaces and roughen them with a fine hacksaw blade. The leather surface should be soft and furry, like velvet.

Clean away all the dust.

Now comes the difficult part. You have to keep the surfaces APART until this is finished. Coat each surface with contact cement and let it dry. When it is completely dry give each surface another coat of contact cement. Let them dry again.

If you let the surfaces touch while they are wet you will have to clean them off and start again which will not be easy.

When both surfaces are completely dry. Press them together REALLY firmly.

They should stick immediately. If there are any SMALL places where the rubber didn't stick right to the edge touch them up with Copydex.

Now you are ready to dubbin the leather or treat it with goose grease if you have any.