Well ... lots of thoughts come to mind.
My favorite single tube pen at low cost is the Zodiac from ExoticBlanks.
That style is based on the Berea Twist Sierra and there are many variants of that basic style, but for my money, the Zodiac is the best value.
I also like the original Berea Twist Sierra and the Berea Click Sierra (different length tube).
I also like the 30 Caliber Bolt Action from PSI, and the Vertex Click from PSI (you can buy both of those at ExoticBlanks too).
For a long-barrel single blank, you can go for the Berea Long Boy and replace the two tubes with a single tube, discarding the center band and compensating the length of the barrel accordingly. . That makes the single best long barrel in the world.
Eventually, you might like to make a single blank long barrel by modifying a slimline, or better yet, a Cigar. . Tutorials for doing both are in the IAP library.
I am just in the process of making several dozen single barrel Cigars, and I am having a barrel of fun ... that makes a wonderful pen if you have the experience and tools to do it. . I did not follow the tutorial instructions in the library, but made up my own way to do it.