Nice idea for a prototype.  What is your plan for the full scale version?  I mean what size, shape, ASIC configuration are you targeting.  Using custom 3D printing is a good idea to reduce excess fluid usage.   What materials are you considering.  I was thinking of that but my concern would be an interaction between the 3D printing composite and the working fluid.  Too bad there are no 3D printers which can "print" stainless steel.   As an alternative I was thinking about maybe taking the 3D design and slicing it into layers and then cutting each layer with a CNC.  The layers could then be stacked inside the chassis.

On edit: I see you are looking to cool a MB and GPU combo not ASICs.  On the pressure issue. You may want to weigh your prototype with a high accuracy scale over time.  Under pressure Novec escapes most seals pretty well so you likely are encountering fluid loss.  Weighing the entire system over time is a good way to measure that rate of loss.  Improving the relative performance of the condensor and keeping the pressure differential as close to 0 as possible will reduce fluid loss to a minimal amount.  It may not be as much of an issue with your small prototype and low energy load but with a larger heat load and larger seal surface area unless you design it to be hermetic the fluid loss rate might be unacceptable when under pressure.