This method works quite well for up to 10 BP clutches with the container store shoe boxes and a small modification to hold the boxes up above the water. I used light diffuser cut into strips and siliconed them to the long sides so I didn't need to use that particular size box. https://www.youtube.com/watch?v=eUIRtUFjgfQ

I've done the broken fridge route too and that works quite well also, though there is slightly more variant in temps between top and bottom shelves. Simple as taking a broken fridge, lining the back with heat tape on a thermostat, and tossing some full bottled water in where there is extra space to add a heat buffer.