We did the balloon thing in chem as well, we used something called the Hoffman(?) apparatus. It collects the gases in chamber so you can let it out with a valve that worked pretty well. Makng something similar to one of those is probably your best bet, maybe you can borrow one from your school, if they will let you.
Also I think our teacher added H2SO4 as well to spead up the electrolysis but seeing as you have metal in there thats probably not a good idea.
~Scott