I fear that I may open up a kettle of worms with this one. In order to produce a truly perfectly precisely accurate straight edge most machinist manuals will tell you to make 3 straight edges, and there is a process to “prove” them against each other…
I’m not working in metal, I do not need to be accurate to 1/10,000″ over the span of 4 feet.
I need it to be OK over the length of the straight edge.
So I probably should have made this post before making the post about the winding sticks. Because making a straight edge is like making one winding stick. (But one winding stick by itself is useless 😉
I have been using the same straightedges for decades, one @ 2′ long made from a fall off of birds eye maple and one made from plane maple but @ 4′ long.
After planing to thickness use your longest plane to make the edges as straight as you can. Use the trick of sighting down along the edge to “see” if it is straight.
Occasionally the edge gets dinged and I just re-plane it.
Putting a hang hole in one or both ends is a good idea too.
And you could make it a “notchy stick” by making cyma and cyma reversa cuts in the end. (a reference to Christopher Schwarz’ notchy stick)