Just a simple block to go between serial connections. I use it between a DB-9 cable and a USB<> Serial adapter. We remove the thumbscrews and use a 1" machine screw to hold the parts together.
This block just fills in the space and makes it look better.

Screws: 1" x 4-40 Phillips Pan Machine Screw from local hardware store.

I printed it on a Replicator2 in low-res, 10% infill, 20mm layers.
Update: v3a adds a little taper to it, and readjusts the spacing of the screw holes slightly. Makes for a smoother fit.

This thing was made with Tinkercad. Edit it online https://tinkercad.com/things/hiM1WoEcB6h

Made to fit between serial cable connections that don't use hex nuts to hold them together. Use some 1" machine screws to secure the cables and use this block to fill in the space. Helps to keep down the "wobble" between the connection and looks better.
Printed on a Replicator2 at 10% Infill, 20mm layers and 2 shells, using warm grey makerbot PLA. Matches my beige cable nicely.