and make it look at least passably convincing?
and still be cost effective enough to not set me back as much as 50+ gallons of milk would?
and not be too damaging to either the tub or the model in it?

water and baking soda... really anything that turns the water opaque...

right, opacity is good.
i want opacity.

Why not use a couple of boxes of powdered milk? It's cheap and it will make her skin all soft and purdy.

Water and baking soda???   You mean baking powder??  I have also had a desire to do a milk tub shoot, and wondered how it is done, if not 50 gals of cream...   I bet baking powder or corn starch might work---actually corn starch in water is considered a newtonian solid.   It is VERY FUN to play with, has characteristics of both liquid and solid depending on the amount of force exerted on it...  somewhat like quick sand

https://modelmayhem.com/posts.php?thread_id=3159
https://modelmayhem.com/posts.php?thread_id=19273

I always use Elmer's Glue

photograph a minature tub full of a glass of milk and edited it