To save going to any expense as you already have a stanley knife...
Stick masking tape along the sheet where you will be cutting....draw your line on it.
Get a straight metal edge and place it along the line...
Score along the straight edge with your stanley knife...a few times.
Now do the same on the other side...applying tape etc.
Place the sheet along the edge of a table or work bench and apply pressuere directly on the scored line.
When it snaps you will have a clean straight edge...!