Whenever I have to cut down a sheet of plywood I reach for one of my trusty metal stud straightedges. Metal studs are available in different thicknesses for different applications. A 20-gauge non-load-bearing drywall stud (about 35¢/ft.) won’t deflect significantly when it’s clamped to an 8-ft.-long sheet of plywood.
I bought 3-5/8 in. x 9-ft. metal studs at my local drywall supplier (look in the Yellow Pages under drywall). Home centers usually stock lighter 25-gauge studs, but they can special-order the heavier gauge for you (25-gauge studs deflect about 1/8 in. over 8 ft).
I trimmed one stud down to 8-ft. 6 in. for ripping and cut the other stud to use for crosscutting.