This is a simple socket set rail that I made for 3/8in drive sockets. It can interlock end to end. The spots for sockets are farther apart the farther down it goes. This fits 8 sockets, ~8-16mm is the best fitting from my experience. There are also 6mm diameter holes, 2mm depth, on the bottom so that magnets can be inserted. This helps keep the item in place in a tool box or allows it to be stowed on the side (cannot guarantee sockets will stay on securely if placed sideways) 
If desired, the STL is available and the distance from spot to spot could be adjusted for larger sockets.

This rail is quite similar to a number of other rails that already exist honestly, but I wanted to be able to create my own, so here it is!