Re: the HDPE. I've only ever had stuff from Direct Plastics, but the black HDPE definitely machines differently to the white natural HDPE. I guess that it's something to do with the dye in it. There is not real difference if you are working it with standard wood working tools and I've never notice any difference between the two in combat.

I don't know much about the different grades of HDPE, but some people use UHMWPE instead which is a little stronger but considerably more expensive. I would say that its far more important to make sure you attach the pieces of HDPE together really securely, than it is what grade they are made of.