Koiczar's right, in that the no-niche skimmer is easy and relatively cheap.
But if you are determined to make one, here are a couple of links to another forum which discuss two different types of DIY skimmers. I can't personally vouch for either design since I've never made one, but it looked like a good place to start. Hope they help.
http://www.koiphen.com/forums/showt...ty+sink+skimmer http://www.koiphen.com/forums/showthread.php?t=30318
Barb