So I made a mistake. The reason the listing for the light I bought was £10 cheaper then the one linked is because it was literally a light, and not a pair. So that is going back.
But I installed one of the mounts (bought separately) and rested the light on it to see how it looks. It is a bit big but also not too big. I am not really sure what I think. Probably need to see it lit to be sure, but I think it worth swapping for the pair as it is not too bad.
I never factored the height when I tested with my cardboard circle, so it will have to be centred on the mount and overhang by 2". It would be better if it could be mounted downwards, but that would not work as it has a lip on the top.
Meanwhile the mounts are very strong and chunky, but I am not sure how well they fit. The inners for the narrow setting are too narrow, and the middle ones are too wide. One of each just about works, but it still twists by hand easily.
They are clearly designed for big lights though, very much overkill even for these. But it is difficult find ones for mounting vertically in front on a vertical pipe. I think I will have to continue looking for those. The crash bars are 27", incidentally.
And completely unrelated whilst I was taking photos, this is my previous, successful, lighting project.