Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:3352266.
This is a work in progress for those of us who use Sonoff Basics combined with regular light switches to make it into a cheap smart WiFI switch. Here is a video that explains it: https://youtu.be/ab472a40-co?t=369 Basically, you're wiring your regular light switch to the low-voltage GPIO on the Sonoff. Then the light switch controls the relay on the Sonoff which controls the lights.
I printed out the model and it fits my Sonoff Basic perfectly. I know that there are multiple revisions. This one was designed for "Sonoff Basic R2 V1.0" but looks like it would work with the newer version. I haven't tested whether it fits well in an electrical box yet, but I don't see why not.
Please take the necessary precautions when working with high voltage, and use this thing at your own risk.
Link to buy Sonoff Basics on Amazon (affiliate link): https://amzn.to/2SMushj
Edit 1/29/19: I put it in a single-gang junction box and it fits. Had to trim down the built-in button, but that's it. Also, make sure that the ground wire isn't going to short anything out.