For a one piece slimline read this: http://www.RedRiverPens.com/articles and go down to #18
Actually, make the upper tube 1 inch long which will just fit over the transmission. Using the complete tube will make the pen difficult to twist and to get the pen apart to change the refill. I need to make that change in the article
Actually, I assemble the nib, lower tube, transmission, and refill and insert it into the blank which is drilled and cut to the proper length. I insert the upper tube and push it onto the transmission just enough for it to grab and hold. I then cut off the end exposed from the blank. One inch works for the kits I use but an adjustment may be needed for the kit used by someone else.
On the nib end insert a section of a slightly larger tube and glue it in place. I use the refills for the purfume app woodcraft number 149764. The slimline tube will just slide in. The transmition will be held by the upper slimline tube (which is also glued in).
Doing this will allow the nib to turn easier and pull out easier to change the refil.