Inner covers of good quality are made of cedar wood. The basic design consists of a framed flat plank with a precut hole in the center of the plank. The inner cover resembles a shallow tray (with a hole in the center).It is mainly used for ventilation.
Look for cedar wood when buying an outer cover. These woods resist rot and last the longest. Outer covers assemble in a manner similar to the inner cover: a frame containing flat planks of wood. But the outer cover typically has a galvanized steel tray that fits on the top, protecting it from the elements. Alternatively, there are some plastic models on the market that will never rot. Not quite as pretty as wood, but perhaps practical.
Reference material:《Beekeeping For Dummies》
