We have a halfords roofbox that is about 8 years old now. One of the places where the U Bolts go through to attach it to the roof bars has cracked. Is it safe to repair this with glass fibre or similar ? I'm just a bit concerned that if one of the mounts has broken then others may also be weakened, is it better to be safe and just buy a new one.
I had this happen on a box a few years ago and like yourself I was anxious that the remaining points would break.
I considered glass fibre, but sometimes this doesn't adhere to well and where you have movement, can peel away.
I made up some rectangular steel plates approx 100x50mm which were drilled to allow the clamp to fit centrally lengthways and then rivetted these into the base of the box to reinforce each mounting point. This worked a treat and the box is still going strong, albeit now used by brother-in-law.
Some boxes seem to be designed to have the 'U' bolt threaded out from within the box (i.e. nuts facing down beneath bars, with a saddle clamp across) and some with the clamp facing upwards with the nuts inside.
The box I had was supposedly to have the clamps facing downwards, but I considered this rediculous as anyone with a spanner could simply whip the whole thing off. Once the plates were fitted, I cut each clamp down and fitted facing upwards into the box. Secured with original nuts, then further fitting of some dome headed nuts ...
... to protect the exposed thread from damaging box contents.