I got pretty close to done with this before I realized my setup isn't going to work. The idea was to have a brushed tricopter powered by a tiny whoop board. This is sized for a 9g servo because that's all I have at the moment; Unfortunately, it's also a 6 volt servo, and almost no brushed quadcopters are 2s. A low-voltage servo would work fine and fit right into place, but a low-voltage micro or sub-micro servo would work even better, and I've parameterized as much as I can in the fusion 360 file. Moreover the only board I can feasibly do this with is brushless, so this was all dead in the water to begin with. I've got a low-voltage servo on the way, if that works with my setup I'll finish this, but with brushless motor hookups instead.
bill of materials:
The tail mounts directly into the servo, and the servo screws into the frame with the supplied screws; you can secure the servo to the frame and the tail to the servo with a little super glue to be sure. Don't permanently attach the tail until you've had a chance to get the servo working as the servo has a midpoint; you don't have to get it right on the money but you want to get close. You might have to resize the motor holes or sand / shave/ bore them out a bit, the idea was that they'd be press-fit.
In order to get this working in Betaflight, you'd have to use resource mapping to repurpose a pin to work for the servo. I'm going to attempt to run my low-voltage servo directly off the battery if I can, you might be able to run a servo with 4.8v specs off a 5V pad but be careful, servos can have large amp spikes. There's something about not remapping a pin with the same timer as the motor pins that you'll have to watch out for as well. more info is here: https://github.com/betaflight/betaflight/wiki/Servos-&-SERVO_TILT-for-3.1
I still haven't done any of this stuff, so take it all with a grain of salt.