Thanks! It has been a fun project! The carbon rods are 6mm and 8mm from amazon or ebay, I don't remember. The 6mm OD rod fits perfectly within the 8mm OD rod with no sanding or modification. I don't even need graphite. The tolerance is just tight enough to allow the rods to slide easily with no play.
It was a bit of a task to assemble and line everything up. The aeropoxy I used has a good pot life which made life a little easier during this process. Keeping everything aligned and concentric was a task that began with the construction of the sustainer motor mount. It was critical to ensure that the 8mm motor mount rods were properly aligned axially or the transition rods would be off and bind during separation. The other error would be that concentricity would suffer and the two vehicles would not be aligned axially. It turned out well. Maybe I just got lucky, but as far as I can tell any which way I look at it or roll it, they look to be inline when mated.
The plastic transition is a flimsy piece of crap from apogee. I reinforced the inside with 2x2 carbon twill and laminating aeropoxy. I lightly sanded the inside with 80 grit and layered strips of carbon. The resultant transition is surprisingly strong and concentric. I then cut the top off and built the rest of the transition (coupler, rods, bulkheads, etc.). The void inside the transition will work perfectly as the chamber for the separation charge. No centering rings were used in the sustainer, so I had to seal the spaces between the rods and fins with epoxy.
I used leftover 8mm rod for sustainer conduit which worked out well. I'm working on the electronics as we speak and will have it in primer soon. Can't wait to get this up in the air!