This is my modified version of the housing for the DIY-VO2max-Device over on instructables.com/Accurate-VO2-Max-for-Zwift-and-Strava/
All credit goes to the users there: "rabbitcreek" for the initial design, code & the instructions, but also "urissel" and "ivorhewitt" for their contributions! I merely dabbled with my CAD to neaten-up the hardware a bit (which I only achieved partly, I think)

In short: This device measures the oxygen your body utilizes while exercising, which is a helpful metric to control training or determine your performance capabilities. In my opinion, this is the greatest thing since sliced bread and the only project worth wasting filament on - but apparently (for some reason), not everyone is a part-time, unprofessional endurance athlete and mad about suffering on a bike...

Known problems with this version:
1) Weight & weight distribution - my design is pretty top-heavy, so you'll have to tighten the straps to an uncomfortable level
2) Water tightness - on several occasions, water/sweat leaked into the casing and messed with the sensors and/or the screen
3) The CO2-sensor suggested by "ivorhewitt" is not included yet

So there's work to do! Feel free to tackle those problems. I'll attach the assembly as a step file, so you can modify/extend/improve the thing to your heart's content!

Update 08/23: I recommend checking out 'gristleking' on GitHub (you'll find the link in the reference section). He's embarked on an ambitious project to improve and enhance the existing design in almost every aspects!

No

No

Print the Venturi-Tube standing on the outlet, so the diameters come out accurate and the tube is air tight. I'd recommend to print without supports since they would be difficult to get out of the channels leading to the pressure sensor. Only in the port for the oxygen sensor supports could be helpful to ensure roundness (see picture 4), but that depends on your settings and print quality.
The other parts are pretty straight forward and should print without any problems.