I use a boom SSQA attachment made by Titan.
The boom is 3" x 3" and 60" long. Note the mount is parallel to the surface of the plate, the plate is heavy, full frame and strong.
I also have a UA V2 Receiver plate similar to the OP photo. I would be concerned over making this into a boom for 2 reasons:
1. The pole would mount at 90 degrees to the plate creating a lot of twist forces to the receiver mount which was designed for a short ball mount only.
2. The forces you are putting onto the tractor front end loader. Look at the specs for lifting capability at the pins then at 24" there is a big drop in capability, if you make it a 5' boom you may damage your loader or cylinders.
The idea of putting a hook in the receiver mount is a good one as you are at the pins, the only downside is you are close to the front of your tractor. If I were making a boom arm for a receiver plate I would keep it short, many of us already have these, they are pallet forks, I lift a lot with straps from my forks. An issue with forks and a receiver plate is that the lifting point is at the base of the mounting plate, not useful for many lifting tasks, the link by I7win7 is an interesting use and looks within the limited capability of a BX loader.
I know our machines can do great things but recommend caution with smaller tractors and loaders. If you look at the construction of 3pt booms you will see they have supporting construction to the main boom pole, they are also limited in capacity.
I use my boom a lot on a Kubota track loader, the lifting capabilities are totally different. It is a very useful accessory. Lifting light loads on a home made boom may work out, be cautious as loads increase.