I wrote this block https://www.codeply.com/p/6jbL7Wl3K2 that works perfect for my purpose. A fix-height top bar, and fix-height bottom bar, the middle parts (left and right) are filling the remaining space.

However, I am struggling to make it mobile/small-screen friendly. I want to make the orignal left and right to be vertical now, with a fixed 400px height. I tried to use various media queries, but all have issues.

https://www.codeply.com/p/6jbL7Wl3K2