Refrigerator grow

I’m helping set a buddy up. He has a old refrigerator the “fridge” space he’s using is 22”x25”x43”. He has a “1500w” cob led board from amazon. Which has made some decent results in other setups. 3 gal tote running a dwc setup. A seedling heat matt under the tote and a 6 fan dialed way down. Box temp is staying a steady 72-75f with the light on. The setup is in a in conditioned garage. So the intake air can be as cold as 15f. Will this cause a issue on its own? That cold of air coming in? The intake air is at the bottom of the box and exhaust at the top. That’s why. I recommend a heat mat to keep the dwc setup from getting to cold next to the intake. Also any recommendations for heat when the lights off? We have the fan on a cooling thermostat so it will kick out if the temp drops below 70. But I don’t think the heat mat allow will keep temps up for 12hrs. Thanks in advance.

Could wire the heat mat into some thermostat like a stc1000. You set a desired temp range and it will turn the heat matt on and off as needed to maintain the temp. If you can wrap the Mat around the bucket maybe find some type of insulation blanket you can place around it all to keep the temps stable with slower temp swings and make the mat more efficient?