The sidewalls of a truck/trailer box are constructed from a plurality of panels (10, 12, 14) which are welded together where their vertical edges meet (82, 88, 90). The panels (10, 12, 14) are each extrusions and each includes thin side skins (24, 26 and 48, 50 and 56, 58) and thin webs (28, 52, 60). The top panel (10) may be formed to include an integral top rail (34). The bottom panel (14) may be formed to include one or two side rails (18, 18′ and 120). The regions that need to be welded are made thick enough to be welded. Other regions are too thin to be welded. The overall construction is lightweight and quite strong and facilitates construction of the truck/trailer box.