I have had a couple of questions regarding cold fermenting equipment for the home and recently posted this as answer to an email question. Thought it could be informative post for all.
To make a cooler for cold fermenting you have to have a dedicated refrigerator or a small freezer with a temperature controller installed on it. If you use your household refrigerator the temperatures will be too high for your perishables. I bought this from Amazon:
Drilled a hole through the back of the frig and inserted the sensor, then caulked the hole (could have just placed the sensor in through the door but the gasket would then leak.) Be very careful if doing this as you have to identify where the evaporator coil is so you don’t drill through it. If you are interested in doing this I would go out and buy a cheap small apartment refrigerator. If you have an old regular size refrigerator or freezer you are not using you can do the same thing. Kinda big though for us home bakers.
I had a small compact frig that came with an outdoor kitchen I bought and never used, so that received the controller treatment.
There are plenty of reasons why your bread could have deflated, too many to go into not knowing ALL the facts not that I am some kind of expert. I tried a 100% whole grain myself yesterday @102% hydration and it did not come out as expected but it was only a 150g loaf of bread so maybe I should not have expected too much height. Crumb very open though.
I did though make my first 100% wholegrain sourdough (after about 25 or so tries) where the crumb was perfect at 80% hydration a few days ago. I did use the cooler for this and first “retarded in bulk” for 8 hours @45 & 48F set points and after shaping and placing in a basket a “slow final proof” @48 & 51F set points for 12 hours. Turned out in the dutch oven and baked cold.
Instead of jumping from 80 to 102% on the hydration I should have gone up in smaller steps at a time. Previous to making this 80% hydration sourdough I did a trial run with a small measured amount of starter,water @80% and flour and let it rise at 80F in the proofer to find out just how long that particular mixture took to fully ferment which I believe helped me.
This was a small loaf it should have been at least double the flour for the dutch oven I used though.