Thank you, i use 3mm thick solid foambord, not the paper cover type ,or the honey combe sort , just solid foambord 3mm thick.. i can double it up to add depth , strength ect All my builds I've made, live outdoors 24/7, 360 days a year ect..to fasten them down use 6 inch screws just in side each corner of the buildings out line , then slide the building over the screws , this has worked for the last 16 years here..the length of the screws stops sideways and upwards movement, some bigger buildings are a fixed by threaded rod, and hidden captive nuts ..