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Someone like MagCad would could probably 3d print a go pro mount with a recess easily enough, but they might not have enough interest to warrant it.
I've done one off work with cat3bits, it was fun to produce something but it would cost in the region £25-35 for one. Perhaps if 2-3 others were interested it might be worth it but I'm sure I can fashion/bodge/hack a solution for free.
I reckon the M3 bolts would work fine - they will surely be strong enough for this? - but you'd need to ditch the PDW-supplied M5 retaining 'nuts' on the inside face of the mudguard, and use an M3 nut/retainer instead.
I think the main issue is that you need to remove some material from the existing go pro mount so that there is a 'tunnel' for the PDW arm to go through (ie you need to drill a hole in the go pro mount where the brand name is on your pic). Someone like MagCad would could probably 3d print a go pro mount with a recess easily enough, but they might not have enough interest to warrant it.