Here is a tool that I use all the time when building models. Everyone should have one and thats the end of it. There aren’t the cheapest tool that a model builder can get but it’s an invaluable tool. Every scenic designer should have one in their tool box. Here is a blurb from Kevin Kelly from Cool Tool …
This handheld tool allows miter cuts from 45 to 135 degrees of small pieces of wood, plastic, rubber, and even metal. As a miniaturist and model maker, this tool has been invaluable for cutting one or a myriad of small parts at various angles and sizes — to change the angle you slide a self-indexing metal guide that runs perpendicular to the blade.