Don't boil cider, it will release the tannins and the stuff will never clear.
You've got a couple options here. You can either add a gallon of 100% juice or pastuerized cider to the secondary when you rack, or add a filler. If you add juice or cider to secondary, you're going to have to give it a little extra time, as that will just start fermentation back up again (unless you want it carbed). your other option, filler, can be anyting that takes up space. A number of people have mentioned using sterilized glass marbles to take up the extra space.