chunk of plywood or similar (Britannia)

I've had this bed frame for many years; but only now, in my old age, has it occurred to me that I could perhaps create a more comfortable headboard for it!

I'm looking for a chunk of thin plywood, or some other rigid substance – 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