The cedar is great. leave it as "ceilings" in the lockers, it repels moths, and so forth.
Lots of ways to make shelves, simplest is to add cleats
fore and aft and use plywood
for the shelves. Level of finish is up to you.
However, we like having hanging locker space, for foulies and cold weather gear
, and for "dresses" or long pants (stuff you wear when going out to dinner or checking in with Customs
in foreign ports).
I feel blessed that this boat has many drawers, and generally speaking prefer them to shelves; however shelves use the space better. Your choice, your boat.
Good luck with the project