I think the factory edge is pretty straight. Now, the question is how do you get the inside edge equally straight? Well, you have two strait factory edges. Buy some excellent ply to start with, not the cheap stuff that will splinter on you. If you're using a circular saw, work face down. Place painter's tape along the cut line, preferably on both sides. Now, cut the sheet down the middle as straight as you can. Now you have two straight edges to work with. Cut a couple of wooden blocks the width of the final straight edge you want, adding the offset for the saw blade. My 18v saw is offset by 1 1/8 th inch, so if I want a 6 inch wide strip, I'd cut the squares 7 1/8th inch.
Lay the blocks onto the edge of the bottom piece, butt the straight edge of the other half up against the blocks. That's going to give you a good cut line. Clamp it together and make the cut being very careful to make the cut straight. Draw arrows toward the factory edge, otherwise you'll forget.
Now, you have two straight edges. Use them to trap the saw's base and your cuts will be very straight. Cut as many straight edges as you want. Do trap the base, it is really a challenge to cut a truly straight edge without trapping the saw.
Now I have a track saw, but this is how I have done it before the that, using my nice little DW 18v saw. I have an 8 ft aluminum L beam that cuts a fairly straight line, but it is not a perfect straight edge.
Last thing, if you can get a premium saw blade, full kerf if possible, or if your saw will accept it. Hope this helps.
The more I do, the less I accomplish.
Last edited by DesertRatTom; 02-19-2018 at 09:27 PM.