I was going to use some strong 3m double sided fixing tape stuff to attach my di2 external junction box to the underside of my BB. It seems due to the shape of the box that there isnt enough contact area for it to stick so i need another solution. I cant use the bolt as there is no hole in the carbon frame so could i just use a big splodge (technical term for the ham fisted) of sugru? Does it stick pretty well to carbon? Any other ideas i could adopt and i am not averse to locating it somewhere else if its better?

If it doesnt stick well you could always mould the sugru to a shape that then lets you use double sided tape each side of it to stick to the frame and then stick the box to it.

Any chance of using a cable tie to secure it to?

Put it above BB?
One of those little frame bags?

Never heard of sugru but stixall works under water.

Can you use the bolt to attach a plate which could then be glued or zip-tied?

Mine is stuck on with a splodge of mastic (PU40) and then I used Sugru to mould a sort or fairing around it. Been like that for over 2 years.

Got any pics of what you need?

Sounds like you could do with some kind of 3D printed cradle or interface bracket?

I could potentially model something up, but would need accurate dimensions and some pics….

I’d fix it above the downtube/BB to protect it from everything flung up by the front wheel. I’m sure that they’re IP-rated, but it’s one less concern. Nylon cable ties would be practical and neat enough for me