Round styrofoam, available at any craft store, black fabric hot glued. Used toothpicks and glue to stick the ears on. Then string attached to each ear with hot glue to hold the thing up.
I ditched the styrofoam in this design. The pins didn't stick very well. I went cork board backing laid on top of styrofoam. Pins seem anchored now. Might take you're idea and cut it out of plywood and glue cork to it
All of them look awesome!
OP your board is huge, don't think dh would let me do something that big, lol.
The kids and I all each just have a standard framed cork board in our rooms. We might have to do something more after our next trip, we are planning on doing lots of trading.