A coiled blank defines two elongated parallel bands and a parallel web separating the bands. Opposite ends of the web form diagonally opposed spacer strips arranged such that, when the blank is coiled to bring the ends of the web into juxtaposition, the opposed spacer strips define between themselves an open triangle having a base lying along an edge of one of the bands. Preferably the triangle base lies along the band of greater thickness in the coiled configuration. A second embodiment provides three parallel bands and two webs. When the second blank is coiled, the second web also forms an open triangle with exposed base between opposed diagonal end spacer strips at its juxtaposed opposite ends.