Since you say finish is not an isue I don't think you need a straight edge. Cut a pattern out of something hard, thin, and light like masonite for a template. Throw itt on top of the sheet of 1/2" and mark your lines out. Then plunge cut.
However, the description you give of how you plunge cut scares the crap out of me. You should never, and I mean not ever, hold the entire saw above the sheet and then dropit into the cut. The way to plunge cut is to tip the saw forward onto the front edge of the sole plate and then ease it down into the cut. Having the front edge sitting on the sheet gives way more stability and helps the saw blade enter the cut square so that it doesn't bind which can cause a kickback.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.