I made some enquiries about the fibre glass spraying and it seems to be
costly.

Someone on a net forum just suggested painting with hammerite. Would
that
waterproof and strengthen, or just waterproof..?

Are the boxes weak? trying to spray something on them to strengthen isnt
going to help in the long run.

The Boxes have obviously lasted quite a few years and are still very solid.
They won't be used outside, but may get exposed to rain/weather whilst being
transported to/from events (wedding receptions/hired halls/dinner&dance).

So was thinking of waterproofing and either covering with carpet or
hammerite to preserve them whilst they return to me a tidy income.