I quickly tried using tape the other day but found when you put enough on to fill the gap, you couldn't actually slide the sections together past a couple of inches and by the time you removed enough to allow said, you were back to quite a gap again. ;-(
I still think a small band of fibreglass at the key points could be a workable solution but I had another more mechanical solution / thought.
What if instead of the M10 bolts that currently located the adjustment by going though both inner and outer sections (and in some instances, also hold other brackets in place), what if I drilled though one side of both tubes say 20mm (the outer is 50 x 50 box, the inner smaller obviously) and turned up some suitably long x 20mm OD, 10mm ID spacer tubes that could be fitted though said 20mm holes and then stop up against the inside of the opposite inner, when nipping them up it would now pull the 'inner' section hard up against the inside of the outer section, whilst the interference fit of the spacer would also keep the other two tubes aligned on the opposite side?
This is the same process I've used many times when fitting towbars to cars where the only fastening was to the bottom of a hollow box section and they provide suitable spacers in the kit to avoid you crushing the box (drill bolt hole right though, open top one up, drop spacer into hole etc).
In the case of this trailer, you could actually run it with all the nuts and bolts finger tight as the insertion of one tube into the other seems to be sufficient to provide the strength (and in the case of the axle halves, the turning resistance) so my only real 'concern', is the opening up of the holes on one side from M10 to M20 (say) ?
I understand the stresses are at their least in the middle of the box sections (especially when the holes go though horizontally and most of the loads are vertical) but what does the panel think?
My goals here are to retain the ease of adjustment / takedown (you can then carry it in an estate car / MPV or even on a roofrack / boot) but to remove any of the clanking, especially when the trailer is lightly loaded.
Cheers, T i m
 The spacer and suitable washer could stay on the bolt, effectively forming a bolt with a bigger shoulder.