Has anyone got any other ideas?

Build a frame out of timber. This will be simple, if you choose the right
thickness of wood, you could just use a thick batten to gain 60mm. Apply
plasterboard. Paint. Rejig cornicing and skirting.

Cost? Very little. Probably change out of twenty quid. Tools required? A
saw, a hammer, a screwdriver, a drill and a paintbrush.

It would be much stronger than using a thin piece of plastic.

Christian.