Hi Alessandro,
You have done well with your cardboard box soft boxes. Did you get your 'inspiration' to build them from the Strobist (
www.strobist.com ) If not, I can highly recommend the site. With the versatility of the swivel heads on the SB800, I encourage you to try bouncing the light off the ceiling or even the side walls. An excellent place to aim is in the corner where two walls and the ceiling meet. It forms a nice 'umbrella' in which to bounce the light. I look forward to seeing your next posting of Marco.