Having explored some ideas with my ready-made boxes I think I could have a workable arrangement.
These boxes (I think I'll call them modules) are 30 x 25cm, made from 1cm thick beech. The thin wood base is inset by 5mm. I'll have to keep an eye on the base just in case it decides to warp, but in the meantime I'll be building my trackwork on thin balsawood on a 5mm foam board inlay.
track on foam board
By happy coincidence the width (25cm) is almost the same as my layout width at the adjoining end, so I should be able to make the transition into this new section look respectable.
The questions now are, how do I join these boxes and how do I carry them?
I found some mending plates in my box of useful stuff, and got some machine screws and wing nuts to fit.
The idea is, that I use these plates as horizontal joints and also as vertical supports.
With the wing nuts tight there's no movement so I'm hoping this will be adequate on a flat surface. (I've no plans to build any supporting legs - I've already checked with the Jubilee Challenge Authorities that a table should be available on the day).
I'll have to get some more modules to be sure how many I could carry stacked like this. I'm planning to use one of them cut down the centre to provide 2 small fiddle yard areas, so if I can stack 5 feasibly the length of visible track would be 1.2m.