If you want a whole piece, I suggest to delete those two planes (they're planes, right?) and replace them alltogether with a pyramid that you have booleanized in the upper part to remove the top so that it fits completely inside the bigger cubic structure. Anyway let me suggest you that for this kind of modeling it would be much better if you learn a modeling program. Wings3D is free, and is a great program to start modeling. It will take you a few days to learn it, but once you do, you will not regret it.