Another example of an external fan mod.
Used one of those "slim" form factor 120 mm fans (15 mm thick). Settled on 5.4V / 1030 RPM after a bit of experimenting. Voltage regulation is achieved by two 24 Ohm resistors connected in series with the fan. Added a 560 uf, 10V polymer electrolytic cap to reduce the 4x RPM ripple that I mentioned above -- might be unnecessary, but I don't want any ripple inside the scope, where it can be avoided.
It is really quiet, only becomes audible when you get closer than ~50-60 cm, and that's in a quiet environment. Temps are ~4 deg.C lower than they were with the stock fan (it's now showing 52 CPU / 48 CPU ambient at 22 deg.C in the room). By running the fan at full 8V the temps are reduced by further 4-5 degrees, but the added noise makes it absolutely not worth it.
Overall look:
Mounting: M4x8 bolts. Anything longer won't be possible to get into the gap, and these are just the right size for this fan. Of course I had to drill four new 4.5mm holes for them spaced 100 mm x 100 mm.
Beware of the clearance between the fan's outer lower edge and the surface on which the scope stands! This is one of the reasons to prefer the slim form factor fans. In this case, the required clearance was achieved with the new holes placed at the same distance from the lower edge as the original ones. This placement also provides enough room for the carrying handle to remain usable, and the part of the fan protruding above the scope's housing is just so narrow that the blades can't be reached from inside, unless you have extra thin fingers and/or try really hard.
And yes, the back lid's mounting screw holes remain reachable with the fan installed. No need to remove it to get inside the scope.
Power cable routing:
p.s. Initially I wanted the fan to suck air out of the scope instead of blowing it in, but, alas, this fan (and I would guess that it's a general rule) is only good at pushing, not pulling. It was totally helpless when it tried to pull air through those vent holes: a lot of noise, very little performance. Pushing air into the scope, on the other hand, works really well.