Here's what we came up with:
We took furring strips and cut them to size. Then we beat them up with screws, nuts, and other pieces of metal to make them look battered. After that we stained them with a walnut/mahogany stain and coated them with satin epoxy so the stain wouldn't rub off.
The look we were going for was that some pirate just came into Roscoe's room, grabbed some old scraps of wood, and nailed the map to the wall so he could look at it while he planned his attack.
While we were at it, we also made a frame for the ship painting that hangs above Roscoe's bed. We started with chair moulding that
Side note: Remember when a few years ago you had to call and try to verbally describe stuff like this, instead of sending a picture message? Remember a few years before that when you had to either guess what they wanted or make a few trips, because a payphone was too much trouble?
After that I cut the moulding to fit the ship painting, backed it with furring strips for structural support, and stained it just like the other pieces above.
And here's the final result, which is a good improvement over how it was before.