Summary

Adapters to convert a geared 28BYJ-48 to a cheap and light direct drive stepper.
There are stl files that allow you to use the original mounting style or convert to a nema 14 mount.
As you would expect from its size and weight, this motor is suitable for very light duty only.
Read the instructions for some important considerations before making this.

Instructions

If you print this in PLA be very sure to watch the heat. The motor could easily get hot enough to soften the plastic.

These motors seem to vary somewhat in their construction so I won't discuss disassembly and you may have to make some modifications to the drive shaft and possibly the adapter.

You can customize the scad file to generate either type of mount. You can also specify the size of bearing you are using. I used two MR105ZZ bearing stacked together to give better side force resistance. If you want a standard shaft you can use pretty much any 5mm id bearing(s). The bearings are intentionally a tight fit and have to be pressed into place. You might have to adjust the clearance for your printer.

Since I am using PLA, I drilled holes in the case and used screws to retain the adapter; but I would recommend a higher temperature plastic that you could glue in place.

I recommend that you also cut the connection as shown in the pictures to convert the motor to bipolar operation as described by:
http://www.jangeox.be/2013/10/change-unipolar-28byj-48-to-bipolar.html Of course you have to use a different driver if you choose to do this.

The shaft is bored to have a tight fit over the original driving gear. It is attached with epoxy. This works even though the gear is nylon because it fills the space between the gear teeth. I extended the epoxy down the nylon shaft and on to the rotor to reinforce it. Be careful not to get any epoxy where it will interfere with free rotation.
I have included a stl for the shaft, but I made a metal version. I think it would be rather difficult to get a printed version to work. It might be possible to use tubing.

Direct Drive 28BYJ-48