Go to a toy store and buy a few bags of marbles (unless you still have your collection from childhood) that'll fit through the neck of the carboy, sanitize them as you would any equipment and slide them into the fermenter to increase the volume inside, decreasing the head space. Glass marbles are easy to clean and sanitize. Be careful not to drop them into the fermenter so they don't crack themselves or the carboy.
You could also boost the size of your batches to 6 gallons.