Such good news to read. Casper has shed which indicates growth.
I've only been keeping T.ordinoides (Northwestern Garter Snake) for a short time but have found both males and females docile and engaging.
Casper is a good looking young female.
How old is your heat rock? Older heat rocks were dangerous and have burned many a snake.
I have heard that the newer ones with newer technology are much better at controlling the heat.
I don't use any heat rocks just to be safe. I use under the tank heat mats or Flex watt heat tape on my larger enclosures.
I have also heard from those that keep Northwesters that heat isn't necessary in more situation. Much of that decision depends on the temp of your home.
As you can also see the heat rock cord gives Casper access to the top of his home.
Garter snakes are escape artist and considering what a rare find she is it might be best to eliminate all possible escape avenues.
You might check into purchasing an aquarium with a sliding/locking lid. A 10 gal. should work for quite a long time.
It looks like she's in a"critter keeper". I remember a forum member posting photos of a snake trapped in one of the lid vents.
I hope Casper continues doing well with you.
Please keep us posted on her progress.