I used tentest and covered it with black fleece. Then mounted it permanently on the wall.
Agree ... important for your base to be pin-friendly, so I would nix the idea of plywood.
The intent is to not need pins on a design wall.
With the fleece I have put queen sized flimsies on the design wall, without pins.
However, if I am leaving it for any length of time, I will put a few across the top ... just in case!
I don't want to risk coming back and finding it on the floor!
Plus with the size of my design wall, I often pin to it patterns, notes, etc.
So pin-friendly is important IMHO!
You didn't mention size ... take note ... you can never have it too big,
nor have too many design walls!
Have fun planning your dream room!