Many thanks for the new responses; much appreciated.

GB, a deflationary bang might actually add to the effect. The possibility
of an inappropriate slowly-collapsing-balloon sound is a little off-putting
(though I appreciate that it would be more likely to be a drawn-out hiss, as
you say). Also I do need a near-instant vanish rather than a slow collapse.

Owain, two lazy tongs at ninety degrees is a nice idea. Garden trellis had
occurred to me though from memory it's designed for one-off expanding and
installation rather than for repeated smooth up and down action. It could
be a good budget source for materials though. I'd never heard of Bilofix:
looks intriguing.

I'd also never heard of the Blue Room Forum, which seems like a splendid
resource. Thanks for that.

Incidentally, is there a source for inflatable figures other than the
obvious one?