I have been forced against my will to earn a living and work has been busy. However, I did dig the bag of bok choy seeds out of the seed box and they are ready to go in the ground tomorrow morning
Without seeing your pictures (I'm at work and photobucket links don't work), I'm wondering if you've sealed the gaps between the water bottles sufficiently -- perhaps you put straw between them..
Another thing to ponder would be to add a 60 watt light to the equation.. I suspect that little amount of heat would be sufficient to help raise the temps quite a bit -- perhaps put on a timer to run at night to keep the temps at bay.. Perhaps that's not practical..
There is straw between the gaps. Temps seem to be pretty stable...between 36 and 45 degrees. I could run a light if I had to, but I don't want to and think I can go without if I have enough thermal mass.