Not sure where you are located but here is one of the better used equipment dealers that might have something that fits your needs.
https://lostcreekmachine.com/
Tip for future use, when mounting heavy/awkward assemblies cut the head off of a bolt with the same thread pitch as the tapped hole or use all-thread, mount the part using the "stud" as a third hand while you start bolts in the remaining holes. Noticed when you mounted the backstop there were no...