How about something like this.
https://www.etrailer.com/Specialty-Trailer-Hitches/Ultra-Fab-Products/UF35-946401.htmlYou can bolt this onto your existing hitch, and then put the bikes on this receiver. It is only rated at 200 pounds, but probably can exceed that for a while.
They also sell straight piece that you can have a local welding shop attach to your hitch, or the welding shop probably has some 2-1/2" tubing sitting around and can weld that to your hitch - I had this done on both sides of my motorhome, one for a dog cart, the other for my bikes. I use the factory center one for the car. The new ones are on the outside, near the side mounts.
Fred.