Plane Board Basics 

Just use an 8'' x 4' standard mdf sheet. Don't bother with the top half though (ie above hip height), otherwise you'll have to swing on one plane which is unnatural. The hole size doesn't actually matter-although it looks about 4ft in diameter. Attach an arm to the rear of the board so you can adjust the lean angle-this should match the shaft lean at address of whatever club you are using.

by the way, I actually think this is genius. From parallel shaft at the back to impact to parallel through the swing, the swing is one plane. I think as you go farther back in the backswing, the plane changes as the club comes up more but one the way down it joins up with that plane board. This is really cool. I try and use this as a swing thought rather than a physical board but if I had one I would give it a go.

Can you give any info on the dimensions of the board? Diameter of the hole, angle of the board to the ground, etc.?

the girl's comment at 0:12 ...