Up until today, breaking an ice tray open over the top helped. The top layer is a moist newspaper. The risk from using ice is the amount of moisture in the worm bin. Fortunately that moisture is helpful in this arid climate. Often the top newspaper layer has begun drying when I put ice inside the bin.
I only put the ice on one half, so if it is too cold for the worms they can move to the warmer half.
Today the newspaper is moist when I check yet too warm. The new thing I’m trying now is placing freezer packs on top of the newspaper layer. There is a tray of food between the worms and the ice. They can dive deeper in the bin if they get too cold.
This has never been a problem. The worm bin has never been too cold. It has to be less than 40° F for it to be too cold.