It turned out that I can fit the power adapter inside the case without modyfing it. Original adapter's power socket will be used.
Main part of the casing is made of 1mm aluminum sheet

The smaller surface will be attached to the acrylic, holes on the opposite side are necessery to use a screwdriver.
Adapter in casing:

Acrylic part will be used to attach the casing to the back panel and main mounting plate. Power adapter fits really tight (it's really hard to get it out)

I want to make back panel out of two parts:
-acrylic which will support HDMI, wifi antenna and power adapter casing
-panel made of an old laptop on top of the acrylic- so the everything should look better
For now only air exhaust cutout is ready