One method of prestretching rubbery polymer sheet prior to thermoforming involves pressing a mechancial shape, orpluginto the stretched sheet. Current popular methods of predicting local wall thickness of thermoformed sheet—finite element analysis and analytical geometric analysis—have not addressed mechanical prestretching. A relatively simple geometric protocol, based partially on the Fukase blow-molding scheme, is proposed for 2-dimensional parts. The protocol is used to illustrate the effect of depth of plug penetration on sheet reallocation in 2-dimensional corners.