Designed to hold in acrylic sheet windows of my printer enclosure, but useful for holding any sheet material in place by its corners. I designed this around Halloween, hence the spiderweb theme, but it is useful all-year round.

The attached STL has 100-mm arms and 3.2-mm screw holes. The attached SCAD source has constant values set right at the beginning, which allow you to modify all the geometry attributes (size, thickness, screw point parameters). The screw points are strengthened and they have a counter-sink to receive conical-head screws.