There will be an endless debate over fermenting in a plastic vessel. But that shouldn't sway you. If you are comfortable with a plastic fermentation vessel, go for it. I don't believe many people lager in plastic, but someone will correct me if I'm wrong.
The bigger issue to me would be getting air tight. A cooler lid has a lot of contact space to seal up to ensure a good air tight seal for long term. Additionally, I don't know how a cooler does under pressure. I imagine if you could get a good seal and drill a hole to put in an airlock it would work fine.
Let us know how it goes...
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