Supine Shoulder Extension - Hands can't touch each other

Hi. I'm unable to touch my hand together behind my back with straight arms.

When I try to touch hands together, my elbows are bent, arms pronate and shoulders are forward:

When I force my arms to be straight, my hands are far apart, but shoulders look better:

Although coach says in the video that not much is happening with a wide grip, I do feel a good stretch in the shoulders in this position. 
Is it okay for me to keep with this form and let it gradually get better?

HI Stefan , have you tried to sit up higher on a block?

@Stefan Broeder

You can also try performing this exercise with the form in your second image but holding a towel between your hands. Over time you will be able to narrow your hand position until they are touching.

I also started with wider position… I felt it TONS… then week by week, I was getting the hands closer. And then they were touching, so I was going deeper.