Getting all three is a mission impossible.
Really fast means a rotary mitre saw equipped with a dedicated ultra thin blade, used in combination with a template ruler (which you have). Too expensive for hobbyists.
Accurately means using the template and a dedicated mitre box with a pin matching the notches. That's cost effective and straightforward which saves time but it's not super fast.
The cheapest way is both time consuming and relatively difficult. The tools needed are an accurate ruler showing the smallest increments, a protractor, a pencil and a saw. Draw the center line, measure the locations on the centerline, draw the perpendicular lines with the protractor... Double check that your measurements match with the table you're reading the measurements from. Triple check that you're reading the table correctly. Wipe off the markings as you notice you've been reading the wrong scale. Redo the measuring and drawing and double and triple checking. Finally take the saw and cut the edges. Recheck that the scale is right. Redo the measuring and drawing process if needed. Cut.
There's no shortcut.