It appears that multiple pieces of conduit, or larger conduit are the ways to go.
1″ conduit is a little smaller (in surface area) than two pieces of 3/4″ but only around 11% (approx). And it might be cleaner and simpler.
Are there any plans for a 1″ conduit mpcnc? Perhaps even just for the middle section?
Maybe I should forget filling the conduit or waiting for a 1″ version and just go with what Fernando is doing…