or even milk jugs. just put black plastic around them or paint.
i made a smaller one last year. something i got had a bunch of
the ice packs with it. so took 4 of those and wrapped in black
plastic. fits in a 10/20 tray takes up 1/3 of it. worked very well
with a dome in the greenhouse. so if you have room get some
water warming during the day. best of luck
