how fancy do you want them?
I built mine from 1x2 (on flat). They are the perfect depth for HO. A section (or 2) of 3' flex track ballasted on cork makes it look pretty good. I didn't use a door/glass/plexi, but a sheet of plexi could be attached on hinges pretty easily. A backside of masonite kept it all square. Adn when all is said and done, they only stick out 2" (or less) from the wall.
It's not a living room quality display, but it worked for me in my home-away-from-home many years ago. The 1x2 were recycled when I built my layout.