Hi, A mate of mine had two 20ft containers welded together and removed the centre metal work creating it twice the size in width, he then foam sheeted it and timbered the intererer leaving the existing wood flooring down. then he proceeded to remove the doors and replace them with an electric roller shutter he now keeps his motor bikes in he has electric for lighting and the shutter it looks really well and without damp it may be worth you looking at buying a 40 ft and cutting it in half then doing something similar