Taking advantage of the opportunity and some free time, I measured the dimensions of the car stereo mounting. It seems like a small thing, but it can be very useful sometimes. Please don't throw stones at me saying 'why reinvent the wheel', but someone might find this helpful.

Full size

1-take a metal plate of appropriate size (thickness 08-1mm)
2-mark and drill holes according to the drawing
3-cut along the outline (approximately)
4-bend sections along the red line to the left on one plate, to the right on another.
5-done
Attach it to the stereo and put it in the car!