A casing for a portable electronic device, such as an MP3 player, a cell phone, a smart phone, or a PDA, is formed with two or more sleeves that may be placed onto the electronic device and together form a closed encasement for the electronic device. The encasement is formed to enable operative access to the electronic device while the sleeves are connected to form the closed encasement. A back panel of the encasement is removable so that it may be exchanged for an alternative panel. That is, a first back panel may simply be a planar panel that only forms an integral backwall part of the closed encasement and a second back panel may be provided with a belt clip or belt loop, or the like. The different removable back panels may thus be exchanged to provide the device for mutually different functions of the casing.