My only thought at this point is that the knife should be cheap enough that everyone will buy one, and that everyone will buy one at well above the value of the knife so there is a fair amount of money to donate to James' temple.
I think a knife in memory of James is a great idea, but the only real connection any of us will have in this knife to James is the money that is left over. To me, that is what would make the knife special.
My original idea was for a small lock back folder with steel handles that was engraved with the Chai on one side, and "In memory of James K Mattis on the other. Basically the "Wee Pen Knife" that James had at the top of his web page. The knife could be made for $X (?) and sold for $2X with everything over the cost of the knife going to his temple from Blade Forums.
If a Puuko is a better choice, then, well, a Puuko is a better choice, and I will joyfully participate!
Mike