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 50¢/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.