I'm curious about your results because my heat bed has the same slight bow. It even bends my glass. Maybe a good solution is to get thicker glass that won't flex but I worry about losing heat. I haven't tested thicker glass to see if degrees of temp are lost per 1/16" of added thickness. Glass conducts heat really well so maybe it's a non-issue. However, fixing the bed solves all the issues. Let us know how it works out. I'm afraid balsa is too soft to do the job. You might need to use oak.