Here is the final result after rotating my iMac-replica from yesterday. First I wanted to work with tools like scaling and bending, but then I found something much more useful: A special grid which does not only help you finding the right angle and perspective, but also adjusting the shape correctly. Without this tool, it is very difficult to see if everything is in correct relation. Unfortunately, this grid could not help out with adapting the iMacs base. That is because the monitor is a simple rectangle without any curves. - A shape that is easy to adjust. The base, however, consists of curves, which behave completely different while rotating. The result looked very unnatural and unrealistic.
So I had to fully recreate the base considering the new rotation of the monitor, what took me a few minutes. Afterwards I was able to copy the shading of the front-view base to the new one, what saved some time. In the end, I only had to add the edges (which are now visible from the side) and tilt the monitor a tiny bit as a detail.
The grid I used to adjust the angles for the new perspective. At first, it was a little difficult to work with this tool correctly, but after getting used to it, the adjustment is quick and easy.