Inline Shift Barrel Adjuster

One end accepts a shift ferrule (see section below), the other end fits a standard Shimano barrel adjuster. Notches cut in for indexing with the barrel adjuster's notches.

https://www.universalcycles.com/shopping/product_details.php?id=6690 Basically, they are these (Jagwire Rocket II Inline Barrel Adjuster). Note: mine are not designed to fit into the ends of STI levers like this version. If they do, that's a bonus. They are free floating barrel adjusters.

Like my downtube shift cable stops, you will have limited adjustability, since you can't reliably thread the 3D printed material, & the threading in/out is limited by the depth of the nut.