400w SOG/Perpetual Grow Box Design

Arite guys I usually dont post but here we go..I'm hoping to start building my first grow box (3x3x5) with a 400w Hps and I'm going for and Sog/Perpetual grow. But first I need input on my design. I have a cool tube that measures 19.5 inches and a inline fan that measures 6 inches. Both have a diameter of 6 inches but that is not what I am worried about. With the length of my box only 36 inches how would i fit both inside while having the ability to adjust the lights height? Should i widen the whole box? Input is appreciated and wish me luck guys. This is my 3rd go around with some decent strains and I want to have everything spot on!

Thanks

sounds good,,,maybe you can put the fan outside the box?little bigger always nice too.

I could make it larger but will the 400w still be efficient at say 3Lx4Wx5H?