I picked this idea up from a Reddit user and I am thankful that people try to think outside the box! (https://www.reddit.com/r/Amd/comments/8fvx5w/so_drunken_me_thought_a_atx_board_would_fit_into/?sort=new)
Always found ATX cases to be not very space efficient, so decided to use the Q300l to squeeze in a Z270 Motherboard together with the 7700k. It works only if you find a new spot for the PSU. In my case, I chosed an SFX-L PSU and screwed it into place. The case is really accommodating as it provides basically countless options to mount something in place inside or outside of it.
Pretty happy with it besides the back side of course. I will close the former PSU exit with something to only let the power cable come out.
For those wondering, the 300Watt is sufficient to run a GTX1060 together with the components shown. I tried that by pulling it out from my other system. Hooray for efficiency improvements in our hardware!