I found a source for HEC here in the US, but I haven't been able to find much information on mixing ratios. If I wanted to make some fairly thick gunge, what's my water to HEC ratio? Alternatively, how much gunge could I make with 1 lb of the stuff?

HEC: https://www.chemistrystore.com/Chemicals_G-R-Hydroxyethylcellulose_1.html

Looking to make something approximating this: https://www.youtube.com/watch?v=4ylg5R4G-CM

I would just buy from Messy Supplies, but international shipping is brutal.

It may be somewhat variable, but with the unbranded HEC powder I have at the moment (from a friend over here in the UK), 150g per 10 litres results in a good thick gunge. You could experiment a little in either direction, but that gives you somewhere to start.

Thickness depends entirely on the temperature. Thin when hot, thick when cold.

I mix the dry powder with a small amount of cold water. That keeps lumps from forming. Then I fill the jug with hot tap water. It thickens as it cools. For room temperature thickness, here's my general rule of thumb

pourable but viscous - 1/4 to 1/3 cup per gallon

Thick like oatmeal - 1/2 cup per gallon

Jello - 2/3 cup per gallon.