The Shinwa Foldable L-Angle Slim is a circular saw guide for square and 45 degree crosscuts that folds down to 75 mm (about 3 in) wide to carry. We carry the 23 cm and the 40 cm.
Open it, hook the fence over the edge of your stock and run the saw's base plate along the guide face. The fence locks at 90 or 45 degrees with a thumbscrew, and there's a fence and non-slip pads on both faces, so you can flip it over and work from either side. The body is aluminum with a thin stainless strip on the guide face, and ABS caps keep the aluminum from marking the work or getting dented. A groove along the edge takes Shinwa's slider (sold separately), which fits your saw's rip-fence slot and keeps the cut from drifting, and a strap hole takes a carabiner or lanyard. Pick the 23 cm (about 9 in) for dimensional lumber and narrower boards, or the 40 cm (about 15-3/4 in) for wider boards and shelving. The scale is metric with Japanese shaku-equivalent marks; there's no inch scale.
Shinwa Rules was formed in 1971 from three measuring instrument makers and is based in Tsubame and Sanjo, in Japan's Niigata Prefecture. In 1977 it received its first patent, for technology to mass-produce calibrated stainless steel rules of stable quality, and it describes itself today as a top-selling maker of metal rules in Japan.