If you need to model something for a lightbulb to screw into, look no further!
For some reason I needed to be able to screw a lightbulb into a barrel (https://www.thingiverse.com/thing:1857863). No wires or power, just a bulb. I couldn't find a good way to do it, so I created this blank to use as a negative.
Take any solid object, like a barrel, place this where you want your bulb to go, and do a 'boolean difference' in meshmixer. Voila, the perfect fit for your bulb to screw into. It should fit most common bulbs, but it's designed for the most common, an A19 (first photo).
Here's a handy reference to bulb sizes, because if you're anything like me, you had no idea what an A19 was.
If your printer is better than my cheap one, you may want to scale the model down a tiny bit. This negative has a 62 mm bulb, where an A19 only technically needs 60mm.