Thanks for those suggestions. I've since found some square nuts which are a good-enough fit and managed to get them tightened up. If this does not work in the long term then I might try the square plate method with threaded holes as I already have a piece of rectangular bar the right size.
I don't have a piece of square bar of suitable size to make T bolts, but if the square plate method doesn't work then I can weld extensions onto the heads of some bolts to make them lock in place like T-bolts.
Zebeddy not available in Australia, similar local products like Unistrut, Ezystrut etc have channel nuts that might suit if the 'home brew' doesn't work.