Summary

I needed to check the elevation of a building site to see if it was flood free.
I quickly designed and printed this sight to press onto a spirit level I have.
It is bigger than it needs to be but I won't be taking this any further.
A tripod adapter would be the next step.
This isn't a precision instrument but was good enough to show the site was high enough. The house being sighted is just above flood level and the test site is a little higher.
I repeated the sighting several time including some with the level reversed. I always got the same result within a 1/2 a meter or so.