I have a Mont Bell that is light and I have glued velcro on it and my sleeping pad to keep it from wandering around the tent. The Mont Bell pads have mating toggles to accomplish that. The $20+ dollar price didn't impress me. Exped has one that looks like a better shape to me.
I have been looking at the Cocoon Ultralight Air Core pillows and I think that is the design to copy if you are going for a MYOG.There is an air bladder with a layer of polyester batting and a fabric cover. You get the support of the air bladder, the comfort of the batting, and the stay-put qualities of the fabric. The lightest model is 3.5oz, so you have your work cut out for you. If you buy one, they run about $25. They have a 6.5oz model that is more like $20.
A Platypus bladder looks like a good start to me. One of the 1.8 liter models are more square than the 2 and 3 liter ones. A Platypus is pretty tough and you may be able to find other lighter sources for an air bladder. Adding a padded fabric bag to one of the Flexair pillows sold on BPL is a reasonable alternative to me.
It would be great to get some multiple use-- a fleece stuff sack that you can put a Platypus in-- that sort of thing. The idea of having to empty a water bladder every night doesn't seem practical, but it does provide a good backup water carrier in a pinch.