The ASOK Jango is 1:1 scale for Temuera and is much smaller. In fact the the Rubies Jango 2 piece is the same size (the story goes it was made from a slightly modded casting of one of the movie helmets, which explains other than some of the toy feartures why it's such a better sculpt than anything Ruies has ever produced.)
I would definatly say you're needing the Don Post sized helmet. As for the M-DSB M/L, it's actually a hair smaller than the Don Post so, it may not be the best fit for you. Additonally to that, M-DSB is now permanantly disbanded. M will not be allowed to sell on Mercs. So, until DSB's life settles down and he's able to recast his own work, the helmets are effectivly off the market save what's already out there.
I know finding the Don Posts is starting to get harder and harder. They are popping up on ebay less frequently and here even less than that. I think between completist collectors and costumers, the supply of them is getting diminished.
One thing you could try if youre unable to find one would be to print the WoF templates at 90% instead of 100%, as there's approximatly a 10% size difference. Then once it's assembled, you can play with going up or down in size.