What’s the best way to stop a 2006 Prius 12V battery from draining?

welcome! with the parameters you've given us, i would say you're between a rock and a hard place. would trading it for a different car be out of the question?

certainly, you can try pressing the sks switch under the steering wheel to off when you're leaving.

 

I would uninstall the 12v battery and put it on a plug in trickle charger inside the house while gone for all that time.

 

another thought is a portable jump starter, but letting the 12v die between visits will kill it in short order.
disconnecting the neg cable is the best idea, but then you have to crawl in through the back seat to get to it.
or you could connect the jump starter to the jump point under the hood, allowing you to open the hatch normally.

 

See if getting a solar or wall charger and using long term (secure) parking while your away is cheaper than paying to have dead batteries replaced at your garage parking spot. Some storage unit businesses allow long term vehicle parking (usually RV's); might be cheaper than parking lots.

 

Most convenient for me, and no memories lost, is to leave the battery as-is, hook up my smart charger (at the jumpstart point, under the hood). Let it run to completion, then just leave it on. A decent smart charger will can be left on thus, gives it a very slight boost from time to time. This is my strategy now, when the car's been sitting 3 days or more.

My pref's, but there's a lot out there, and it should be around 3~4 amp: CTEK 3300, or 4.3.

 

there's no power where she parks. and no sunlight for solar.