Vasoline is your best zippo friend. Take out the insert liberally coat the insert with vasoline, slide it back into the case, wipe the excess into the cracks to seal off the gaps. I did this trick and my pipe zippo is on its inaugural fueling for 2 months now.
I have heard this before, but I am Leary of the vasoline seeping out in warm weather. I agree that would not be an issue this time of year!
I also heard about using tape to get the same result, but that the fluid puts a hurt on the tape adhesive.
Maybe Zippo will one day modify the design a bit and add an O-ring.