A magnetic saw guide. Originally I made this with a laser-cutter from wood, but found it was not straight enough.

With this, you cut perfectly square every time without effort (see pictures, I could have made that cut blindfolded).

Obviously this will not work with saws that have tooths that are sticking out to the side or have anything else that prevents them from being laid flat on a surface. I use it with my Ryoba.

It is made for right-handed people, but you can just mirror it for left-handed use.
The handle makes it easy to even grab small rectangular pieces or even rods. I sometimes even use it in mid-air, but it's easier if you stabilize it on a bench.

The triangle hole is there so you can hang it somewhere. The biger holes are just there to reduce the amount of filament a bit and make it lighter.

This has been designed with 4 strong magnets that are 12mm diameter and a height of 2mm. Keep in mind, that even the magnets labeled as strong can have very different strengths. You might need to adjust the model to have more or less distance between the saw and the magnets. The saw should stick to the flat surface, but not create too much drag, as that makes sawing more exhausting. It just needs to guide the saw. It doesn't need to be able to hold it in place. The model has a variable #magnetDistance for that purpose.

I glued the magnets in with superglue gel.

Made with onshape: adjust to your own liking if you want