I FOUND AN UNINTENDED GLITCH AND GOT THE SECRET!! But I won't say what it is because spoilers.
Ok because you asked nicely. I shall put it below.
So in the first room with the box, go to the corner to the right of the door. Box jump into the corner and once you reach the corner's ceiling, try to fling the box up still facing towards the corner. Fail. Fail. Fail. Fail. Fail. Look up a walkthrough..... FAIL
Once you do that enough times at a correct angle you should clip through the wall and ceiling and you can land on a piece of wall. Then you have to fling the box out, and just not letting it go and flinging it around randomly should work. Once you get the box out, carefully box jump to the later rooms, staying above them and to the right a bit. It may take some time finding where to land, but the secret area is really big so you can find it. Once you are to the right of the later rooms, and a little ahead, stop box jumping. (Not too far to the right but not to close.) Then there will be some audio, and you found the golden cube. If all that didn't make sense look up a walkthrough of the secret ending.