I would use dirt with sawdust or those wood shavings on top. Then if it were ever time to do a refresh of the bedding material to move my compost out to a garden bed, it would provide the birds with hours of entertainment scratching out all the larvae and beetles from the base of the pile! I totally X2 Lazy on the required skirting because those insects really aerate the soil beneath the compost, even on my reinforced concrete(like) ground!