Thought about the number plate bracket and made one from the 3mm road sign but felt that it would vibrate too much particularly at about 200 mm away from the mounting points.
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Decided I needed some section within the design to provide a bit more rigidity, thought about folding the edges but the attachment at the top need to fold rearward to allow a flat surface to bolt to the frame rail vibration mounts and the lower part needed to fold forwards to provide a flat surface for the number plate to attach to. Inevitably there would not /could not be a fold where the two surface met and this would be a weak spot.
I had some 8mm aluminium tube so I thought about how I would bend it (having annealed it first).
I knocked up a 9mm x 3 ply sandwich with the two outer layers describing the outline of what I wanted for the outside diameter of the tube and the inner sandwich filler being the inner diameter. This took some time and I had to wait overnight for the evo stik wood glue (really good strong stuff) to set. I found a longish spring with an 8mm bore and proceeded to try and bend the tube. With the spring in place the tube did not quite fit in my jig, also to bend tube effectively you have to go past the line you want and allow the metal to spring back. My jig was effectively scrap so I just bent it by hand with the spring around the tube. It came out not quite (but almost) square, given that there were 7 bends required in 3 planes I was quite chuffed.
I then made up some 3 x 2mm fillets to provide a mounting plate for the whole unit (1) and the number plate (2).
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I then proceeded to "weld" them together. Where the two ends of the tube butted together at the top I cut a strip of 2mm sheet into a thin strip and forced it into both ends of the tube , then pressed them together. I clamped the fillet strip in and clamped it together with some aluminium clamps and heated it up. With the clamps on in the past the heat needed is extensive before the filler material flows so I thought I would get it good and hot. With steel you can see how hot it is it goes from cherry red to orange to almost yellow. Aluminium just continues to look like aluminium. When I applied the filler rod about 1omm either side of the join just collapsed as the aluminium melted - I could have cried. The only consolation is that it will be right up under the seat hidden by the rear light, but I will always know it is there