I built the Mother of Fermentation Chiller, which is an improvement to the Son model. Go for it. It was a fun build, and works pretty well. I haven’t put the weather stripping into mine yet, so I suspect I could get even better performance out of it.
That said, if I chill my wort down to pitching temp, I can maintain a fermentation of 55 degrees by switching out ice tubs every two days or so. It will eat through the ice pretty quickly once the fermentation starts rocking, so be prepared with a couple changes of ice.
I’ve fermented two batches simultaneously in mine and still maintained ale temps of about 65 degrees. If you don’t keep an eye on the ice, the temp will get out of control and it’s hard to bring it back down with ice alone. So the key is to chill all the way to your pitching temp, and then keep a close eye on the ice and switch it out when you need to.
I plan to trick mine out with weather stripping and better latches soon, and then we’ll see how it does with a lager. However, I’m sure it would only work for a lager as far as primary fermentation. You’d have to have a temp-controlled fridge or freezer for the actual lagering period.