take a soft drink bottle and cut the top off just before the neck starts, turn it upside down and place it in the remaining part of the bottle.
This is a picture off the net I presume its collected water over time, I keep mine dry so I have live flies to feed out instead of drowned ones.
Easy and cheap, I used old fish bait and ox heart trimmings and left it in a warm part of the yard, within a couple of hours there were lots of flies. Tape can be used to seal it better but I found sprite bottles it was snug enough.