Hi Franck,
impressive system !! which backplane are you using ? looks like you have 12 slots, is it active/terminated ?
agree with your comments regarding the fan : I must admit I left that aside, and I am only now considering to mount one, which is a bit too late... Honestly, my current internal setup is V1, i.e. aimed at debugging my first cards until I move to a larger / active motherboard. I think it will operate with the top off most of the time so that should be OK.
Then, my plan is to rotate the cards by 90 degrees, i.e. parallel to the case sides, in that case I think mounting the fan on the rear panel would make more sense. My biggest concern is noise : the fan I purchased (AC 220V) is quite noisy despite it advertised at 32 dB. If I decide to mount it, I will probably add a switch or a speed control (I found very cheap AC dimmers on eBay)