I won't insult you in saying to look at a list of woods to determine which are 'hardwoods', but I will say that as a general rule, if your fingernail makes a dent in it - it ain't hard. But then are different classifications of hardness, etc. I can make a dent in some maple, but not in rosewood, purpleheart, etc. You can use anything virtually anything that can support the tension of strings pulling on a screwed in bridge. This topic comes up very frequently around here and I'm still trying to figure out how to make one out of cardboard just to be a smartass For testing out - you could always use what most of us do which is pine, ply or mdf.