3/4" Pex tube seems to have a lot of potential for me.  Initial observations: it can be machined out to fit either 3/4" 0.750" which is widely available, as well as .706" which is 1/2" conduit.  I want to stay true to the build and keep it cheap/affordable so will pursue the conduit, but very optimistic for PEX in 1/2" and 1".  I see 1" being good and bigger/better than 3/4 but the widely available conduit and 3/4 pex off the shelf keeps them a main focus.  The PEX ID is 0.671, I bought a 45/64 drill bit which is .703, and then I got an 18mm reamer which is .708.  I also got an adjustable reamer which ive never used, so that should be interesting.  Another neat part of using the 3/4 pex is its outside diameter is 7/8 0.875 and it plays well with 7/8" tooling.