well, the machining would only be the cap that sits on a 40mm. the tubes and such could be a bunch of old used stock barrels. though, clear tube would be best but not essential. the top reservoir could be any plastic container with holes drilled in the right orientation then all glues or epoxied together. add a lid and holes for the plunger to fit through and bob's your uncle. making it all solid so that it doesn't break when you ram the plunger down is the harder part.
I can only load and charge 3 kings in about 5 minutes with a 200 round speed loader. I should look into getting one of those big mag sized ones.