Sorry can't help with your quest . I had one for my last horse and have to agree it did feel a bit like a plank , but it was a great fit , horse was happy and I loved the position. It was very heavy too.
I've got one! It's a black, 17.5, 29 width one. Yup the seats are hard on them but once it's shaped to your bum it's remarkably comfy, have hacked for hours in this one - it's quite a modern one so maybe they are more padded. Love the Stubbens, they fit all of my lot really well. Pm if your interested