I am trying to build storage for a panel van. My problem is how to make use of pre-existing holes for attaching bolts to. The loading space in the van has vertical structural beams which have holes that I would like to use to secure the storage. The holes are shaped something like this:
It is not easy to reach behind those holes - basically only via other similar holes some distance apart. Otherwise I might have used a particularly large penny washer and put a bolt through it. The distance behind the holes to the van's external panels is approx 25-35mm depending on location.
Is there a trick that professionals use to attach things to these holes? I presume that's what these holes are for in the first place?
The only alternative I can think of is drilling new holes and using a tap to thread them to accept a bolt directly.
Edit: Thank you for everyone's suggestions/answers. Lots of fresh ideas which I will pick out bits of and combine for my solution. Could not choose an individual answer, so up-voted all helpful contributing answers!