Take a glance at
Eurekazone.com to see if they offer anything of interest to you.
I have used their tracks for several years. First rate. Very happy. Note that their tracks are TOUGH - like I-beams. Not flimsy like most. Also the connectors are self aligning due to the dovetail shape of the grooves.
To start I also bought one of their "EZ Ready Saws" (the Hitachi, but you can mount most any circular saw you might have). Last fall I finally made the jump and purchased a used EZ-ONE Woodworking Center.
For starters my advice would be to get their 118" two-track system which I think ships with one 54" track and one 64" track. The 64" track is great for cutting across the 4' width of plywood. The combined 54+64=118" track is great for cutting across the 8' length of plywood. Attach your own circular saw to the included base.
If you want to buy a saw with the base already installed, then it may be cheaper to buy the 54" & 64" track lengths, 2 outside channel connectors, and 2 regular clamps separately.
Cut on foam or consider their multi-form table setup (I think you could create your own less portable version using 1x's with a kreg jig).
Don't hesitate to call them if you have any questions. This is a small U.S. company based in Florida - not a mega corporation. They are great people to work with.
The top surface cuts are VERY clean since the plastic track edge and the saw base act as zero-clearance edges for both sides of the kerf. The bottom are clean because circular saws cut upwards (unlike table saws). Cuts are VERY accurate because you place the track edge where you want the cut kerf edge to be. No guessing.
I'd tried the homemade plywood "tracks", but they didn't guide the saw enough for me ... and the saw still went off course. The tracks are perfect every time.
OR consider their Saw Edge Guide instead of the tracks - maybe that will do what you need instead. Also clean and accurate. I just bought one myself but haven't used it yet. I'll admit I'm a little nervous to not use the tracks.