Pretty simple to make. Build some wood frames, add a little swoop to the top piece for design and then do as "NYCgrrl" suggested. Once they're done, add some "L" brackets on the inside to mount to wall. Most of the manufacturers use two washers welded together at a right angle instead of the "L" brackets.
Sounds like a fun project. If you can afford it, add some MCD shades to make a nice update. If you do, make sure the new valances are at least 4" deep to fit the shades. Do a few at a time.
Don & Mary
2019 Newmar Dutch Star 4018 - All Electric
2019 Ford Raptor Crew Cab