Custom gazebo or jmavsim airframe

How would one go about making a custom fixed wing airframe in either of these simulators? Essentially i want to validate a custom airframe ive physically designed in sitl or hitl? I’ve successfully ran both simulators with their default airframes

You have to create a sdf file (for gazebo) for your costume airframe or modify existing ones. There are tutorials on gazebo website or you can read existing sdf files to learn how to write one. They are under Firmware/Tools/sitl_gazebo/models

If you want to further use PX4 to control it, there is a lot of work ahead of you.
You also need to modify the mavlink plugin in gazebo and control output in firmware to match with your airframe control channels. That is my understanding so far. I have not made my costume UAV flying in Gazebo yet.