Do a search for threads by wizardofflight in the armor section, he posted some very nice blueprints.
There is a great PVC tutorial on instructables.com which won't set you back much money, and are some of the best designed knee armor i've seen so far.
They also use WOF's blueprints, however.
Here ya go...
Remember when you make these, make the UNDER piece all one piece, and not TWO as the patterns elude to, or youll be cracking them in half more than you can shake a stick at.