Hi Toby
I actually scratched a few of these wagons (3 early versions and 2 late) for my layout. The basis of my scratchbuilds were the parkside plate wagoins but to be honest only the soulbars were of use and even then you'll need to take off the w-irons/wheel bearings. You can contact parkside and ask for just the black sprues to be sent to you. I fitted whitemetal roller bearings connected to compensated units for the wheel assembies and added sprung oleo buffers.
There's a set of works drawings for the late version wagon in the Model Railway Constructor 1983 Annual which can still be found online.
Here's a pic of of the early variant Timber P wagons that I made....