So even though the news about this new MAG replica is out, I am still continuing with my scratch build pair.
They are now solid. Filled with a 2 part plastic with a shore of D65. Hard-hats are D80.
http://www.therpf.com/attachment.php?attachmentid=632205&d=1465123825
After watching a heap of videos about lasts, I think I am on the right track. I do need to build a back section that mechanically locks to the front. The 2 parts need to come apart so the last can be removed from the shoe. One of the videos shoes this guy in Japan hand carving lasts. Clearly by the level of skill, he had been doing those for years. Other videos showed lasts as pre-made plastic parts.
There was also a really cool video on the making of a PU upper where they mix a rubber, lay it down as a thin sheet over a machined template and then vacuum degas the mix. Unlike my vacuum system (which is pretty quick at less than 60 seconds), this rig pulls an instant vacuum. I guess in a factory making 1000s of pairs, you need to be able to do that.