re: Spline material

I've found that using "splintery" wood for splines requires some extra care when trimming them to be sure they don't splinter in a way that would cause a problem.

Since the part of the splines that will be visible are end-grain, you might want to test how whatever finish you plan to use will look there.  The walnut splines will probably look darker than the sides of the box.  You might like having a more subtle difference like that.