thin plywood or thich cardboard (Britannia)

I'm looking for a slab of some rigid substance that's 62" wide, and 33" tall – to create a padded headboard for my bed.

Even a fortified cardboard would do – like this chunk that was used in the packing of my neighbour's new TV. It's perfect, except not big enough ... but if I could find a similar piece, I could put them together.

(The plan is to cover the board with foam padding and decorative fabric – both of which I already have).

If you can help with some suitable material, please get in touch.

Thanks,
Antonia