Adding New Vehicle to SITL

I am trying to use the SITL to simulate a Heli quad, but it doesn’t look like that is supported already. I am trying to look into how I can add it myself, but I can’t seem to find any resources how you do it.

This resource says it is straightforward to add new vehicle types, but it doesn’t not go into detail how that is actually done.
https://ardupilot.org/dev/docs/sitl-simulator-software-in-the-loop.html

I tried running a regular Heli and changing the frame type to Heli Quad, but it seems like that causes issues with the way the rotors are set up and it can’t take off.

Any directions or resources I could use would be appreciated.