If you can find "L" brackets that are long enough and bend the opposite end (that isn't bent yet) up or down to make that end just like and identical to the other bent end, you can then drill holes in the bent ends. Attach one of the bent ends to the side of the top portion of the stand frame, in the middle of the tubing or wood beam that the brewing vessels sit on, but below the very top portion that the vessels sit on, (or you can attach the end to the wind shields if you have those). You can weld them in place or you could use a bolt, a washer and a nut, or you could use a tap drill bit and drill a tap hole that leaves a thread (circular groves), that a screw with the same thread size will screw into, and lock the bracket in place. Then on the other bent end (that mounts to you burner) you can drill a hole and then put the bolt that comes with the burners, through the hole you drilled into the bent end and into the hole that is already pre-drilled in the burner.
If you can't find "L" brackets that are long enough you can make your own brackets. You could take a long enough piece of straight metal and bend the ends and make them any length you need. You willl need to make the bent portion long enough so when you mount the burners to them the burners will be four to six inches below the vessels that you are brewing on. Depending whether you are brewing on pots or keggles, the burner height might be different for each.
Most people have the burners mounted between five and six inches below the top of the frame if they are using keggles, to get the right combination of flame height and oxygen for proper combustion, as the burners need to breath and have enough air space around them so the flame gets directed upwards and not out to the sides. If you are using pots around three to five inches from the top of the frame seems to work better for them. This is just a starting point as some people use liquid propane and use a regulator to convert the gas to low pressure for their system, and some use liquid propane in it's high pressure state for their systems. Others still use natural gas with their systems and natural gas has it's own height requirements for proper combustion and flame height.
You can drill several holes in the bent ends from three to six inches (as an example), and then if you have to move the burners up or down to adjust the mounting height, you can just put the bolt in the hole that is far enough up or down from that point, when you test out the burners. A a starting point you can probably start at five inches below the top of the frame for keggles and four inches below the top of the frame for pots. You will need four brackets to mount the burners properly and support the weight. Those Banjo's weigh 30 pounds each and are heavy. This should work out excellent for you though!
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