Nick,
Flat ring spanner taped to a batten.
A small bit of tape in the ring so the nut won't fall thru.
An assistant with electric screw driver.
I also put a washer on the top of the nut with some Duralac which helps keep the washer in place and also minimises reaction between the stainless and aluminium.
Start with the centre holes and work out from each end of the beam.
May take about 30 mins with a good assistant and you will never have to do it to that beam again.
You can also connect the traveller with long pop rivets but will have to put in bolts sooner or later when the rivets corrode the aluminium and let go.

Regards,
Phill

I know that the voices in my head aint real,
but they have some pretty good ideas.
There is no such thing as a quick fix and I've never had free lunch!