Brad, I too have been chucking axes. I used a large chunk of pine, a cookie cut of the end of a log around 24" across, I have knots in mine, these should be avoided. End grain should hold up better than cross grain, would be my first impression. Having a saw mill makes gaining the material needed a snap. I used scrap 2x4's for the 2 legs with a back leg as well, these were tied together forming a base for the triangle. GRK screws of course and an impact driver.

What distance from the target were you using? What were the axes like?

Tim