I'm getting a pair of rigid carbon forks similar to those made by Niner or Enve which are tapered along their length and more box section than round. I quite like mounting light weight but bulky items such as my sleeping and bivvy bag on the forks, max weight of about a kilo each leg. I use Blackburn cargo cages with a bracket from Decathlon on my suspension forks to stop the cage rotating or slipping down but these are designed for round tube. I've looked at the options that Bikepacking.com tested recently but nothing seems to fit box section apart from the King cage jubilee clamp but I'm worried about stressing the carbon. As the forks were offered as compo for a cracked frame, I'm unable to get ones with integrated mountings and don't fancy the consequences of rivnutting these forks myself. Has anyone succeeded in mounting stuff to this type of fork or is it back to carrying this stuff in my bar bag?

8mm cable ties will do it

Given that the forks taper along their length, I'd be concerned that zip ties might slip down. I suppose a bit of innertube or heatshrink between the cage and fork could offer some friction. Ideally I was looking for something more engineered, possibly 3d printed, to give more clamping force without crushing the carbon.

How about these: Zefal Gizmo

https://www.tredz.co.uk/.Zefal-Gizmo-II ... gLxTvD_BwE

I’ve a set on a round down tube and it works very well indeed. Should work ok on a odd shaped fork especially with a bit of inner tube underneath to give it something to bite into.

I've used these on MTB suspension forks and found them very effective. £9.99 per pair from Tredz and seen at the same price on Amazon.

https://www.tredz.co.uk/.SKS-Anywhere-B ... gLuvvD_BwE

Decathlon do a rubber bottle boss mount for about £3. I use them on my downtube to hold an extra cage underneath. Just add a wrap of electrical tape arround the middle to prevent it slipping.