We are a user community, not a support team. 
 

The up-firing speakers in the Arc Ultra need more space than 5cm. 
 

From the User Guide:

• If placing Arc Ultra below your TV, keep 4 in (10 cm) of space (minimum) between the top of the soundbar and the bottom of your TV.

And that’s just the recommendation for clearance on a tv for the upfiring speakers to work effectively. 

Thank you for your reply.

In your opinion, what shall be the minimum space required to have a fine sound ?

Do you think having a coupe of era 300 instead of the bar would be better in my configuration ?

Thanks.

Forget my second question. It doesn't make sense to replace the arc with a couple of era 300.

Not really practical/possible either, with the 75msec delay. 
 

Why not put the Arc Ultra under the screen? 

Thanks for your reply.

Because there is a door / windows behind the projector screen. As the screen is up most of the time, windows is accessible.

you can put on the roof if you want but to get the best it should be as close to ear hight as possible to get the best sound.