I was about to do this exact thing but with AIOs before I decided to go custom. My solution would have been to try and find a 90° philips and screw the middle fans directly into second rad. So each middle fan will have 8 screws, top and bottom. Personally I would have had fans-rad-fans-rad-fans, thick boi.
Here's a product the might help:
https://www.amazon.com/dp/B0016VAIBI/ref=cm_sw_r_cp_apa_glt_fabc_9RT0WWZQD57XX1AM3BSN
Note, you might need to get custom length screws from a hard ware store. The thickness of the fan bezel might not be enough.
Ultra-low profile offset screwdriver. Not only does it fit in the impossible places, but it makes loosening stuck screws really easy. Because it's so slim, it doesn't really try to cam out.
https://www.amazon.com/dp/B0016VAIBI/ref=cm_sw_r_cp_apa_i_UhkBFbM99J1CW
I bought a 1/4" driver identical to the one you linked, and this set of screwdrivers. Drivers are much more low-profile. Don't know what your application is exactly but those might be your best bet.
For a one-off application you could also make your own with a welder and a piece of sheet metal. For that matter, you could probably make a 1/4" driver by cutting a 1/4" hex hole in a piece of sheet metal and epoxying the bit in place.
Low profile screwdriver. Saved me from taking the flaps back off. https://www.amazon.com/Anex-6102T-Profile-Driver-Handle/dp/B0016VAIBI/ref=sr_1_2?ie=UTF8&qid=1539793196&sr=8-2&keywords=low+profile+screwdriver#sp_detail_feedbackForm
http://i.imgur.com/lr2wjGW.jpg there are two screws that i cannot get with a screw driver... Maybe using a very small allen wrench can get them but even that is hard to find
https://www.amazon.com/dp/B0016VAIBI/ref=cm_sw_r_awd_kzybvb173A7CZ i found this buy $18?? Really?
I have a set of these that work wonders in tight spaces: https://www.amazon.com/dp/B0016VAIBI/
$15 might be a bit steep if you're not a tool hoarder like me, but there are plenty of good alternatives for under $10 in the related products lists. Good luck!