My old faithful circ-saw guide has served me well for years.
From this . . . To this . . .
An upcoming project will involve a fair amount of sheetgoods for the guts of the carcass. It is time for Shop Made GCS-v2. Rip a piece of MDF (I used 3/4" . . . wish I would have had some 1/2" around) and a piece of hardboard (I used 1/4").
The pieces are an over-sized 54" long and 3-3/4" wide for the MDF and 9-1/2" for the hardboard. These widths will vary depending on the offset of your blade from your circ-saw's plate edge. I glue and clamp the parts flush along one long edge and go get some sleep . . .
The next morning, using the known parallel long sides on the hardboard as a guide, I run the hardboard edge along the fence and shave the laminated side to assure things are accurate. I also cross cut the length to have known square ends.