Homemade level.

I have been thinking of building a level and am wondering how to go about making the bubble adjustable? At the moment I am thinking of 4 brass screws that get countersunk way under the surface of the wood so there is no chance of them being twisted by accident, Maybe even covering them over with a dowel.

Also what would be the most stable wood that I could get locally (northern Wisconsin) that will stay nice and flat with changes in humidity?

Kari Hultman made a homemade level - it was nice. Search for it on her blog ( http://villagecarpenter.blogspot.com/ ) there's a few entries that discuss quite a bit of how it went together and all, but I don't think hers was adjustable.