Tha last project I did at Voergaard was a pair of pattens. They were made purely out of necessity, as we had a lot of rain that week.
They are made from alder (Alnus glutinosa), which was cleft and worked from the still fresh wood.
Alder will get very light when it dries, so the pattens won't be too heavy. Poplar or willow would also do just fine for pattens.
The most important thing about making pattens is to place the raised wedges in the right position, so it won't hinder the movement of the foot.
They need to be places right under the heel and right where the toes bend when you take a step.
And I really recomend people using medieval shoes (with leather soles) to get a pair of pattens!
They keep your feet warm and dry, and they make a fabulous noise when walking on cobblestone pavements :-)