Resistance across materials
Like the comments have said, screws are not really the best thing to hold the sensor together. You could try taping the thing together and it would probably get better results than the screws. You can also buy screws with a special coating that insulates them. (Spray paint works pretty well if you want to do it yourself.)

The screws will have an impact. I would avoid using them because there are so many other ways to solve this problem. 

Hope that helps!