Need to cut a cube type thinger

Alrighty, here’s me dilemma:

I’ve got this shape, and I want to cut it along this line. Tried the knife tool, but that gave me a really jagged edge, and looked atrocious. Then I tried a python script for cutting it, but that gave me weird results. Any ideas?

make it so that vetically it is only one face deep, then pull up the upper edge slightly, and with the knife tool, make a diagonal cut through the middle using the grid as a guide to get your angle at 45%, then after you seperate the two halves ( i am assuming you are going to do that ) push the upper an lower edges back to where you want them, and remove any doubles using alt M to weld them.

Didn’t quite work exactly as I wanted it, but it’s good enough. Thanks, Modron.