https://www.youtube.com/watch?v=G-wkbElmebk&feature=youtu.be

The Estus flask from the Dark souls series, there are two versions, a smooth one and a textured one that try's to emulate the games design. The models have been leak tested for over a week. The container holds about 6 oz(half a soda can). You can resize up as much as you want, resizing down however might cause leaks as the walls will become thinner. The threads are very tight to help make a good seal, if that doesn't matter to you, you can resize the top to about 2-5 percent smaller than the bottom. One of the tops have supports in the model itself to help save on plastic and to make sure the threads turn out right.

Doesn't Matter

No

As high as you can

100%

If you want to store fluids in this than I highly recommend 100% infill as well as a shell thickness of at least 1.6 mm to help prevent leaking. A raft may help the model adhere to the bed but I don't recommend supports as most software will generate supports inside the flask.