mahogany, oak or birch is fine. Keep in mind that denser hardwoods will take longer to season (absorb moisture)

The use of spanish cedar is not a must. It is used traditionally only to impart the flavor. The next traditional wood used is mahogany but most any other hardwood can be used. Keep in mind that any strong smelling hardwood will impart it's flavor over long term storage. Mahogany, oak and birch are very light in the odor area and are probably the better choices.

My uncle had a solid mahogany humidor that was great. He stored some of his cigars in solid cedar cabinet boxes inside of it and it was plenty to impart the cedar odor to the humidor.

If you do decide to go the cedar route DO NOT USE WHITE OR YELLOW CEDAR! Make sure that the liner is SPANISH cedar. Any other type of cedar will ruin your cigars and technically can be toxic.

Best of luck!