Just use a flat bottomed straight router bit the width of your plywood or slightly less. If needed, you can widen the dado to the right size by applying masking tape to the edge of the router that runs along your straight edge and running along the cut again. the masking tape will move the router over just a little to widen the cut to the correct size..
I'm not slow, I'm pacing myself!
Isaiah 44:13 Another shapes wood, he extends a measuring line, he outlines it with red chalk. He works it with planes and outlines it with a compass...
Usual kit: Table saw, band saw, dual base router and table, lathe, various saws, planer, sanders, and a multitude of hand tools.