Well, first of all, the objects aren't set to convexhull. The convex shape will then form to the correct form that you want. However, the concave shape will not be the correct form. (It would be triangular prism.) So far, this option hasn't been made yet in SP (Although it can be bypassed by making many smaller objects in place of the larger one). So if you make everything correctly, it is possible. This may not be the best explanation, but I hope it helps.
blok in blok tutorial.skp