well, basically my center post is just a seat post i purchased from my local bike shop for about $20. after deciding exactly where it needed to be i cut it to length. on the lower frame tube i put a grade 8 screw with some small washers where the base of the post needed to be anchored. the screw head and washers fit up inside the cut off end of the seat post giving the base a secure mounting point. at the top i made a small dimple in the underside of the upper frame tube to act as a "seat" for the set screw that came with the seat post. once the base slips over the bolt and the set screw is tight against the upper frame it is very rigid. this also allows for a certain amount of pivot which compensates for the frame flex associated with stretch bikes.
awesome ride! i envy the people who can ride these machines legally in their state while people like me (from new york) sit with their hands in their pockets or ride an expensive, no good ebike. once again congrats on your bike build!