What can I say . . . . except I have the greatest followers!!
Other names may be: mount board, book board, illustration board or birstol board.
In Danish: passepartoutkarton and it may be sold in artists shops.
And . . . . someone looked on Danish e-bay and found some for sale in A-4 size.
So, I hope this helps because you have got to start building some furniture!
You all have a great holiday season and I'll see'ya next year . . . . . . . . Kris