I usually cut my mix roughly 50/50, but could probably go lighter on the acetone...the more solvent you use, the more likely it is to attack plastic, rubber, and other materials the mixture ought to be kept from. I would not recommend brake fluid because I spilled some once and took a big splotch of paint off the car fender. Marvel Mystery Oil is probably suitable, but I haven't tried it only because it costs significantly more than bargain brand ATF.
I've kept mine stored in the same squeeze bottle since 2014 and it has held up fine--the plastic does not seem to be affected. You will, however, have to fashion some sort of seal at the lid to keep things from leaking at that joint. I simply cut out a perforated disk from a piece of old inner tube and wrapped the cap tightly with a zip-tie to help make sure it stays firmly in place.