What are your thoughts on the subject? My step-dad and I were just talking about how nice a panel saw would be to have.
That post seems a little bit shorter on details than a person might like. While I strongly considered building such a saw, I finally realized I didn't have a good place to put it, since I would have needed to dedicate most of a wall in the garage or all of a wall in my shop.
What I built instead was a track saw. Mine was made from a long scrap of plywood about 6" wide that I cut a 3/4" dado in. Long dados like that turn out to be hard for me, so the first one was garbage. For the second one, I bought a featherboard to hold the plywood consistently to the fence, and the dado was much more consistent. I also had to clean up the depth of the dado with a router plane, because getting a consistent depth out of a dado stack is a challenge few woodworkers have overcome. The good news is that you can buy a second hand router plane from ebay and new cutters from Lee Valley for under $60.
My circular saw is a cheap one from Skil that I bought sometime in the very distant past. Conveniently in the base plate there were two holes, about 3 inches from the blade. I cut a 3/8" wide strip from a piece of 3/4" oak scrap I had kicking around from a bathroom vanity, drilled and countersunk it, and bolted that to my saw's base plate with flat head machine screws.
The very first time you use this tool, your first pass trims the plywood to mark the cutting line exactly. Make this cut with the plywood track clamped to the edge of something and the saw blade clearly hanging out into space.
I've had it kicking around for a couple of months now, but I didn't get to use it until Friday, when I used it to size up some MDF sheets for a set of desks that I'm making. To make a good cutting surface I put a couple of scrap 2x4s over sawhorses to make a makeshift table. I then marked my cut lines, clamped the saw down with the cut edge on my marks (I only mark at the edges), and made the easiest sheet good breakdown cuts I've ever made. They were clean and straight and my corners were square without any going back to clean things up.
If you want to make your own, try to use a wide enough plywood scrap that it will stick out about an inch or two beyond the motor housing of your saw. Otherwise you'll have a problem with your clamps getting in the way of your motor. I didn't do that, so instead I have to clamp with a small pair of f-clamps from Bessey that have a very low clearance.
Since we can't afford a decent table saw (or even a crappy one if we're being honest here), I did some searching around for ways to use our circular saw to rip sheets of plywood a bit more scientifically than the "eyeball" method we currently use. I found something on woodworkingtalk.com, and am thinking that it would work well for the same thing as your idea. I just wish I knew how the heck to make this thing! This link should (hopefully) take you straight to it, but if not, scroll down to post #19 to the picture with the DeWalt saw....
I also saw mention of using angle iron, which might work too, but I haven't seen an angle iron longer than about 6'. Not to say they don't exist, just that they may not be easily accessible.
I started with a plywood ripping guide like they show there. Those are decent within their rights, and better than eyeballing it. I found that anything over about 3 feet gave me trouble though. As I extended my body out I couldn't register it as easily against the fence and the line would start to waver. The track saw locked it into place and prevented that unpleasantness.