And there are two shots on the left – first one with direct flash, and second – with the home made softbox. The strobe is about a 1,5- 2 meters away and 1 meter higher from the models face, camera left.
The model’s face is lit by softer light/second photo/, compared to the test photo with direct flash above/first photo/ and the wall shadow is moderately soft-edged.
Then I’d tried to test my project without the paper diffuser sheet/third photo/. I didn't exptect much, cause of the reflective surface of the folio inside, but the result is pleasant- much softer appearance, barely can see the shadow on the wall. Notice the different light source reflection in glasses also.
Despite my exptectations, the light is softer when the diffuser is off. Why? Let's see the light source itself:
The light comes mainly form the borders and the reflective folio surface in the middle is darker. The light hit the face from different directions which are opposite one to another. There is no well define shadow on the wall. So my project now looks like more than a ring flash, than a softbox. Anyway I like the final result.
OK, Let's start. You need: - big cardboard box, tape, glue, scissors, reflective folio. You can see step by step process on the following photos:
The size of the softbox depends of your cardboard box. Bigger is better/softer/. The base measure depends of your flash head size. Take a sketch on it. 
The construction contains two mirrored surfaces – the base- contains the side wall and additional spaces for sticking with the front/cover part. Be aware when you cutting the side wall curve insets- if they are wide, the holes will appear in final construction and the light will leaks.
In my case the cover surface is smaller than the base. Cut off the front surface when already got your base stuck. The base and the cover part are both covered with a reflective folio. Use the less reflective surface of the folio. Otherwise you risk spoiling the ring effect.
Putting things together. Place the front/cover part inside the base part, so the additional spaces are outside. So now all of your inside surfaces – base, walls and back of the front panel are reflective. 
Put your flash in the bottom of the softbox and set the bottom mirrors facing it. Stick everything with tape. That’s it!
