Garters enjoy warmth, mine will take every chance they get to be under a warm lamp it even in my shirt when I hold them. I'd say you should have at least a little bit of heat on one side. My carefresh is not dusty at all it's just newspaper chunks pretty much.

Also, how do you know that your snake is happy? I cant tell whether mine are happy or upset, especially when they don't show emotion on their faces.
When I was holding Snap today and giving her a little neck rub she could have been hating it, but not wanting to move... Or, loving it and not wanting to show it. She hates being held and will usually fight to get out of my hands until she's tired.