That's pretty much what I do, I just use empty containers from stuff like cottage cheese.
I fidn my plastic bags get scuffed and they leak if I thaw them before heating...
However, I actually have this adorable little crock pot that pretty much EXACTLY fits the contents of one cottage cheese container of soup, pop it in frozen solid before coffee break at work and it's piping hot by lunch... for stews I have to add a little water and for frozen pasta, a lot of water...