Thanks Mr M. Good idea, but I'm not sure how I'd secure the magnets in the XPS. That did give me another idea though, I could cut a groove in columns of XPS and then carve the quoins into the XPS. And then the walls could just slide into the XPS columns. Unfortunately I don't have any suitable XPS for it or anything to successfully, quickly and easily cut the grooves (A craft knife just wouldn't do it well enough imho).
I do have some 5mm thick sheets of XPS though, and you've given me an idea of how to make a quick and easy base, with a cobblestone-like pavement, for each of these buildings, that can be removed for storage...
Anyhow, after the slight disaster this morning, I put my brain in gear this afternoon and had another go at making the corner pieces...
This is the result -
The new corners are superglued to the top wall in this photo -
Side views -
How they look on the inside, the styrene strip is only glued to the same wall as the quoins, making a groove/channel, it's not pretty but it works, so far -
And it all still comes apart ready for storage -
I'm happier with this setup, although I have some fettling/filling to do on the new corner pieces.
I will probably glue the plastic beam to the new corners and glue the bottom level plastics in place on one side of each wall piece, it'll add a bit of stability.
Some of my Evergreen angle styrene isn't 90 degrees like it should be (no idea why?!), and despite some bending to try to fix it, it ended up making the right corner not square, thus the groove/channel ended up being more of a v shape, so I had to put a bit more styrene in that side to hold the front wall...
There are still magnets in place, but they're only holding the floor level, although I guess they may also provide some stability for the corners as well.
So, just the two big buildings to make, for the first third to a quarter of a table's worth of buildings, I'll have to make all the corners for them (along with making 4 beams), but that should now be a trivial matter.
Thanks as always for looking, C&C welcome.