Mate in the long run it might cause more trouble than good, It's great that you care about the eggs and the ducks, but unfortunatley it's nature.
Anyways, a cheap way to make an incubater just in case you plan on still keeping them (you prob will keep them, lol) is, put them in a shoe box and wrap it in cotton wool, put a lamp directly over them and keep that on, make sure it stays around the same temperature though. Last important thing, make sure you rotate the eggs about every hour, really gently. You need to move the eggs around so that the developing chik does not stick to on side of the egg, or even get their face stuck to the shell, it's importaant to do this because it stops alot of deformities..