I built one for the rear off my frame rail so I could have them above the motorcycle.
I used 1/4" square alum hitch tubing and it flexed too much.
Had to attach it to the frame of the motorhome rear wall with urethane bushings.
Rides solid. No issues yet after several thousand miles.
I can remove either the bicycles or the m/c without the other.
And it comes apart completely in less than 5 minutes.