A portable modular building structure that includes a plurality of sections assembled adjacent one another. The sections include molded fiberboard panels, that optionally include interior plenums. In an embodiment, each panel has an upwardly curving profile, and mirror-image sections are attached to one another via a fastener, that may include tape or other mechanical fastener. The sections are positioned end to end, with the number of sections determining the overall length of the structure. An optional end section of the structure may include wedge shaped sections positioned end to end, forming a curved end.