The invention encompasses microelectronic package lids, heat spreaders, and semiconductor packages comprising microelectronic lids or heat spreaders. In particular aspects of the present invention, a microelectronic lid comprises a material having a rectangular peripheral shape that defines 4 peripheral sides. Further, the lid has projecting peripheral rails along less than all of the peripheral edge. For instance, the lid can have projecting peripheral rails along only 2 of the sides. Alternatively, such microelectronic lid can be described as comprising a generally rectangular shape defining four peripheral edges, with two of the edges having a greater thickness than the other two edges.