Abstract: A modular panel which may be combined with others to constitute a mold for depositing concrete or the like is disclosed. Each modular panel is corrugated to have ridges and troughs which extend parallel to and alternating with each other. At least the troughs are provided with openings in the form of elongate slots with or without circular holes combined therewith such that the openings define nailing passages in alignment with those of another modular panel which is joined with that panel, with no regard to the relative position of the joined panels. An elastic transparent strip is adhered to the bottom of each trough to cover the openings while allowing nails to be driven therethrough.