The classic old Bacharach "Draftrite" uses a simple tube with a straight cut end. See link below.
As to building a manometer, the value you will be measuring is typically .05" of water, pretty tough to measure with a conventional U-tube manometer, typically, an "inclined manometer" would be used.
Also, be aware commercially-produced manometers sometimes use gauge fluids with different specific gravity than water, and won't give an accurate reading unless the correct fluid is used.
Here's an example from Dwyer: