Yes you could make your own manometer, but to make one that you can see the correct amount of draft with is very unlikely.
The correct amount of draft is .05" of water column. So lets see 1" of WC would me this much: I-----------I . If you wanted 0.1" it would be about this much: I-I . So HALF of that would be your desired amount of draft: About this much: II . Seeing that much difference between the height of the opposite sided of the 'U' tube would be very difficult.
So: we have a Manometer loaner program, for those who wnat to know, but don't want to buy and install a gauge perminantly. If you PM forum member Matthaus and supply him with your address, you will get on the loaner mailing list, all it will cost is the postage to send the manometer to the next person on the list after you are done with it. Just a few bucks.
The Dwyer MarkII, model 25 spreads the .o1"wc out over about an inch and half, by tipping the tube on the side. and you can see the amount of draft quite easily. Take a look at the Manometer Loaner thread : Manometer Loaner Program
About the fourth post down you will see a good photo of what you want your draft to be, about .04 - .05" WC
Take care. Greg L.