If you own any Xiaomi devices such as Redmi Note 7 Pro, Redmi K20 Pro, Mi 9T or Mi Mix 3 that runs Miui 11 (which was available couple of days ago), you will learn that you can actually set custom ambient display images instead of new set of pre-defined ones.
To do this, you must have the updated Ambient app installed from here. This method do not require root at all.
Once installed, a brand new + button will appear under select background.
For the best result I’ve used a square PNG image with black background in the size of 512 x 512 px. Don’t know about transparent images, but you can try.
There isn’t much else to tweek apart from clock position, show notifications or not.