Ok, now you have the idea of the box working. I agree with lub that the largest rock needs to sit upright, maybe on an angle. It's easier to do with some sort of soil/gravel in the box. but what you are doing is ok.
Notice how all the scapes you have tried are squeezed into the corner of your tank? Usually this is avoided because it tends to give the impression of less depth/space. Weird as it sounds if you bring the rocks out from the edge and take up more of your tank space it will give you more depth to your tank.
Try placing the large rock around about 1/3 of the way from one side of the tank and just over half way back.
Search Iwagumi in google images and have a look at the ones you like. notice how they all tend to have a larger rock to one side then other rocks arranged around them?
I love your stones too. Keep trying, you will find something that works sooner or later.