I've used ice in a ziplock bag a few times and it works really well for my all-in-one nano. I usually just place it on top of both of the return lines. I think it cools more efficiently since the bag of ice isn't actually making contact with the water, thus staying cold longer- super sciency.
Maybe you could do the same or if you have a sump, wrap the bag of ice around the return pipe with some tape, zip tie or something.