The spring I'm using is a #9 compression spring picked up at the local Ace hardware, measuring 3/8" OD and 1 1/2" inches in length. It doesn't quite clear the post at the top of the guage, so it sits on it somewhat strange. I had to revise the handle to make the spring hole 10mm deeper, and the spring is binding on something, either the edge of the hole, or possibly the two screws, as the screw holes ever so slightly impinge on the spring hole.
All the extra space taken up by the bigger gauge also stretches things out and if I didn't have reasonably big hands, this gauge would be too big. Interested to see if we can make the whole thing smaller.