Skip the ziploc bag. Just put the sour mash in the crockpot. I may be mistaken, but you may need to do your saccrafication rest first to convert the sugars and then mash out above 170F? Once that is converted, cover the mash with saran wrap. That will be sufficient to keep out the oxygen. Then set the crock pot to the lowest setting to keep it around 100-100F. This will be the best and safest way to maintain your temps. Once that is soured to your liking (48-72hrs), place that back in your main mash and proceed as normal.