Large 20×20 Shed Base. Establish a foundation for a garden structure using an EcoBase RapidFit Base Kit. Serving as a green option compared to cement slabs or wet-mix concrete, the Ecobase RapidFit consists of reused frameworks that link together to deliver a sturdy, robust, enduring, and well-draining foundation for your garden structure. 20×20 Fastfit Large Shed Base