I would recommend not using a floating thermometer. It's just a matter of time until you break it and have to dump your wort. Also doubt you'd be able to read the thermometer if you can't see in the kettle.
Please stop calling your homebrew Lambic.
How hard can it be?
Also, just to let you know, I boiled about 4 gallons in my 7.5 gallon pot the other day in my garage. It was about 20 F outside, and I could barely see it