So, I did a few variations and tested them. I was starting to realise why the Cateye guys had made their clip the way they did!

The bottom row shows a progression that was working better each time. By the time I'm at the centre middle, the 'C' with hooks worked quite well. From there I added a springy part in the centre (next to the right) and then revised that part by cutting out spare material. This clip worked well on my two sample saddles so I added the GoPro mount:



Struggling to get this to print at the moment as the fine support areas are not sticking (may need to clean print bed and replace base sheet), but I think it will be OK. I may do some cosmetic changes (not entirely happy with that cut out for extra material).