General Contractor here
Yes they are or can be assembled in place. The units are designed to go through attic accesses in the ceiling of 24" x 24". So double doors would be ok. Code says you need a free and clear work area in front of the unit 30" out.
Be careful not to affect the intake of combustable air to the furnace and water heater if it is gas. Typically a larger vent in the wall or one of the doors works good. You just don' t want to sufficate the equipment. Hope that helps
