The Holy Stone RID unit is $89 at Amazon and works fine. Attach it to the rear of the Inspire with velcro. I've had zero problems with mine and they show up reliably with AirSentinel and OpenDroneID.
For recreational flying, a single RID unit can be registered at the FAA Drone Zone and used on a fleet. For commercial use, each drone needs either native RID or a separate RID unit.
I'm delighted I can keep flying my old Inspires, they're the most badass looking and sounding drones there are. The ZenMuse X5 is very fine. I hanker for more, have no good reason to have it...