I had to remix VegOilGuys design, because my flask is smaller. I use one with 80 mm diameter (Update: 60.3 mm added) - and I wanted to use it for lost-wax-casting with a wax sprue.

You can use 9 mm sprues for example those, that were casted with my mold.
https://www.thingiverse.com/thing:4082778

I first printed the lid from PLA. I worked, but I had to tore it apart to remove it from the plaster. Maybe a release agent would have helped her. But instead I printed the next one out of TPU (flexible). This works great.

Yesterday I did my first lost wax casting and it worked for me. Only the plaster had some cracks. I have to improve the burn-out-process or find better (investment) plaster.

This is a good place to say thank you to VegOliGuy! I build a vacuum-chamber following his video and found a lot more helpful tips on his channel:
https://www.youtube.com/channel/UCkEYj8wtK3aEW8vSGhlB43g

To improve the effect of the vacuum, you could use the „Straw Technique“ as described here by VOG:
https://www.youtube.com/watch?v=5GkLZynNvNs
I use a Flask-Liner - also based on a thing by VegOilGuy:
https://www.thingiverse.com/thing:4132885
I made it customizable for different flask-sizes.

UPDATE: Added a version for a smaller flask (60.3 mm) an the freecad-file.

IMPORTANT: Be aware to print it with 100% infill!
If not, there is air trapped and if you vacuum your investment the air with expand and rise up from the bottom to the top and some of the bubbles will stick to the model.