A cover for the grain tank of a combine harvester is disclosed that, when raised, is capable of acting as a tank extension to increase the storage capacity of the grain tank. The cover comprises side panels (10, 12) and end panels (14,16) of which the edges interlock when the cover is raised. In the invention, each of the side and end panels (10, 12, 14, 16) is formed of a plurality of hollow extrusions (50). Each extrusion is of box section construction with parallel inner and outer walls (52, 54) separated from one another by two or more transversely extending webs (56, 57, 58). Formations (60, 62) on the side edges of the extrusions (50) permit each pair of adjacent extrusions to be interlocked with one another along their length.
