chunk of plywood or similar (Britannia)

I'd like to try creating a padded headboard for my bed, to replace the current metal slats. So I'm looking for a chunk of thin plywood (or some other rigid substance) that's about 62" wide, and 33" tall.

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

If you can help with a slab of suitable material, please let me know!

Thanks,
Antonia