I find the "dead space" in the top actually useful. I use for other stuff like an audio DAC, which otherwise would lie somewhere on my desk. Also getting a custom baseplate from my university workshop for free soon
SFX_L won't fit I think. The steel frame is made so that an SFX like corsair 600 fits perfectly. You can get a D5 inside easily, question is rather which reservoir you want. EK-XRES D5 REVO PWM 100 for example does not fit because the brackets, which hold the reservoir are too wide in width.
Since people have asked: Here's the design made in TinkerCad to mount the Phanteks R160. You can edit it yourself to tweak your screw spacing. My print shrunk a little when it cooled, so I heated up screws to mount the reservoir (I think those are M2.5 screws or M3) and used fan screws on the bottom since they're good at holding plastic. https://www.tinkercad.com/things/7UTTfNuBAv8-phanteks-r160-mount-for-sliger-sv590