[rsync]

"I have a circular saw, but it both scares the piss out of me and is hard to keep on the line."

You should buy a large size "speed square":

https://en.wikipedia.org/wiki/Speed_square

They are thick and have a lip so you square the lip against the edge of your board and that gives you a straight edge to run your circular saw against.

No clamping, nothing complicated - your non-sawing hand holds the square against the board instead of just holding the board itself and then the saw can just run along the straight edge.

I never pick up a circular saw without picking up the large speed square. Here is a video:

https://www.youtube.com/watch?v=tJ95G0hRyFo