Summary

Coupler that fits inside PVC (or whatever) pipe. Half a coupler, actually, so it can be easily printed and then glued together. This one is designed for a slip fit within 3/4" Schedule 40 pipe, and it has a thin raft-like flange around the perimeter to minimize warping. Dimensions are easily adjusted using variables in the OpenSCAD file.

Instructions

If you'll need something other that a 3/4" Schedule 40 PVC slip fit coupling, edit the OpenSCAD file and modify the variables as desired. Grab the software from openscad.org if you don't have it. The variables to edit are at the top in the first few lines.
- pipe_id = the outer diameter of the pipe to fit in the coupler, in millimeters (20.93)
- wall = wall thickness of the pipe, which sets the diameter of the inner ledge (3.25)
- gap = the clearance between the pipe inner diameter and coupler outer diameter (1.25)
- flange_w = width of anti-warping flange, or 0 for none (6.0)
- flange_t = thickness of anti-warping flange (0.2)
Save and compile the OpenSCAD file, and generate an STL file.
Print two of them. I used Cura 12.07 default settings, PLA, with 20% infill. 190 degrees. Each took a bit over an hour for me.
Trim off the flange with a hobby knife. It should come off easily, a minute or two of effort.
Glue them together with cyanoacrylate. If they are a bit warped and don't fit perfectly, heat up the flat faces. I put them in a hot pan until they're just hot enough that they aren't quite sticky, then press them together, and wick the glue into the seams.
Use.

5-Way Inner Coupler