Proof you don't need thousands of dollars in tools to make it happen, and that a good tradesman goes more miles than flashy toys.
I've done similar with a throw away table you smooth up the edge of some dimensional lumber, use some hardy screws to screw the nice edge down to the table, clamp the piece you're cutting down and but the edge of your circular saw up against it, I either shimmed it away from the guide rail or ran a spacer between the circular saw and the guide. Can be tedious at times but definitely works.
Nice little cabin by the way.