Have towed a GV "four down" for over three years behind my motor home. I bought a vehicle base plate form Blue OX and also use a Blue Ox Aventa ll tow bar. Base plate approx. $300; -- adjustable 10,000# tow about $800.
The base plate comes with backer plates that slide inside the bumper frame (hollow tube) after removal of the tow loops for one set of mounting brackets. Another set of brackets bolt directly to the main vehicle frame of the toad. The bolts are about 5/8" thick hi-tensile material and require proper torquing, and thread lock to insure safety. Two safety cables are also recommended to tie that bracket to the frame. These are in addition to the cables between the toad and tow vehicle.
In all, this seems like a well engineered system, specific to your vehicle. I, personally would be reluctant to fabricate any home-designed system. Just the liability issues, should the unit fail while towed down the road, would be enough to frighten me. To kill/injure someone, or have your net worth destroyed after your insurance company refuses to defend you, would be enough to insure that I would buy vs the make option. I like having the assurance of Blue Ox Corporation and their insurance carrier between me and any possible personal liability.
Tom