There's a lot of soft-side zippered briefcases that ought to work. I had to look up the Delorme dimensions (15.6" x 11"). That's a little too big for zippered portfolios. But most laptop or messenger satchels should work just fine and be close-fitting. Still a lot of added bulk though.
Seems like a nice map-case design with a clear PVC side pouch on the top / side of it would be just the thing. Like a lap desk, where you can fold the book open to the page you want and slide it in the clear-view pocket. With the main compartment having room for additional books or related supplies. Any decent seamstress should be able to readily whip one up. If you look at military map cases you can gather lots of ideas for features / details. Don't forget a carry strap.
This design folds up in half, zippered around the edge and the two zippers are tied together in a way that makes a carry handle / lanyard.
