Shop for C-TEK® MODStiff™ sheets. C-TEK® MODStiff™ has a stiffness that is between modified polyethylene and copoly. It is used in applications where modified polyethylene is not stiff enough and copoly is too stiff. Custom cuts available. In stock, ready to ship. Stocked materials typically ship in 2-5 business days. Can’t find what you need? Get a quote.