Free vector drawings of a selection of rulers, set squares and a couple of 180 degree protractors. Loads into Inkscape. The large image can be ungrouped, then the smaller 180 degree protractor re-grouped and copied to a new file. I used the eps format. The post-it notes and pins look nice as well. My Atom netbook with 1 Gb is just about able to manipulate the 16Mb eps file in Inkscape...