So I did the second blowhole & made 2 shrouds. The shrouds have the advantage of giving more variation fan mounting & using the supposed optimum distance between fan & radiator.
The top Shroud holds 3 fns blowing down onto radiator. The bottom shroud holds 2 fans pulling air away from radiator, into the top of the case. For those of you who don't use water cooling , the air coming from the radiator isn't that much warmer than ambient, it's nowhere near the temp of the air coming from an air
cpu cooler. The alternative reversing the flow with 2 fans blowing upward and 3 fans pulling, isn't as efficient. As a rough rule ogf thumb pulling fans are about 50% as effective as pushing ones. so 3xpush + 2xpull = 4fans compared to 3xpull + 2xpush = 3.5fans.
Made the shrouds using a bending press.
Got a roll of neophane to act as a gaskets. Shroud made from 2mm aluminium sheet.
Have made them modular so that assembly & mounting is easier (minimises danger of putting a screw through the rad:lol)
Top 3xShroud Components
Bottom 2x Shroud
Ends Bolt in to Sides.

Sides have mounting holes to Case and Radiator, in line with standard 120mm fan holes.
Intake Fans & Filters mounted to top panel. (has the option of mounting fans internally or externally so giving an option to change distance to radiator
Fitting together
3xside to radiator, fit both sides then screw on top with fans, lastly insert ends.

giving
Mounting 2xshroud to radiator

2x Side showing holes to secure to case
Fitted to case

View from rear of case before putting in ends.

Secured to case
Wanted to save my fingers when playing in case so fitted the fan guards

If ever I don't water cool my
gfx card these intakes should help cooling it.
So that's the shroud now the case