The tank is perfect. Is is custom? If not, can you provide a link? Great rig.
My propane tank is a vintage Manchester 13 lb. (3 gal.) horizontal ASME tank that was often used in marine applications. Unlike current DOT propane cylinders, it does not have an OPD (overflow protection device) and Acme nut. Instead, it has the traditional check valve to prevent overflow and a POL connector. As such, it requires additional care to be sure the valve is closed (no automatic shutoff in the valve), however these valves were used for decades with very few incidents and are not inherently unsafe.
Manchester shows horizontal DOT cylinders and ASME tanks on its website, however they are pricey. Also, the shape has changed and look more like a vertical tank placed on its side (although configured specifically for horizontal use).
You can still find tanks like mine on the used market at old school trailer supply yards and marine surplus outfits. Might take awhile, but they do pop up from time to time.