I have the 64oz Hydroflask Growler linked above, and it’s been working very well for me. It seals very tightly and insulates very well, but that also means that the temperature of the liquid going in stays that way for a good while; I’ve found no good way to cool it externally to any real effect.
In order to start with ice-cold Soylent and have the growler keep it that way, I begin the prep the night before I intend to consume it, and prepare my Soylent the following way:
I add approx. 1250mL of cold water to my blender, and add in a few handfuls of ice, then blend on high speed.
Once I have a nice slush (too much ice and the next step becomes difficult), I switch to the lowest speed setting and add Soylent powder slowly, allowing time for it to incorporate the powder before adding more.
Once I have all of the Soylent powder incorporated, I pour the mixture into my growler, and then rinse the inner walls of the blender vessel with a small amount cold water to remove the residue, and add that to the growler as well.
Ideally I’ll have left enough room to add another handful or two of ice, with sufficient space remaining for the addition of the oil the following morning, plus a small volume of air to help with shaking to allow for enough mixing.
I then place both the growler and the oil bottle in the fridge overnight to keep everything as cool as possible until I begin to use it.
The only issue I’ve had with the growler so far is that I tend to get a bit of Soylent left on the lip and threads when I pour the first few times due to it being so full. I typically just wipe it up with a napkin or paper towel, but it can be a little messy at times.
Hope that this helps!