Catch43,
he used a small Powerade bottle(like 6-8 oz) which has a ridges/ribs on the bottom section. He cut out on of the sections, rounded the corners and put padded moleskin on the inside for comfort. Measured maybe 3.75"x 2.5" after he was done. The ridges/ribs were strong enough for protecting where he got hit and didn't hurt his blocking. Couldn't really tell it was under the sweatbands; he would put it in his back pocket when he was on the bench. He has about 3 of them in his bag as a "just in case".
Personally, I was very suprised at the structural strength of this contraption; I had been telling him that it wouldn't take a hard hit but I was wrong.
Couple of the other players made some after they got dinged and said they didn't really know it was there after awhile.