Need to install a Sonos Amp outside.
Can these be installed in a sealed weatherproof enclosure or do they generate enough heat that they will need some type of ventilation?

Coastal Source makes such an item, I believe.

They SELL that item- it's called a NEMA electrical enclosure. They're widely available with/without ventilation.

[Link: nema.org]

Also, it will absolutely require ventilation.

I've used this in the past for similar applications: [Link: altelix.com]

They make one with mounting tabs for a sonos amp, good company to deal with. We used them for outdoor PoE switches and access controllers.

[Link: altelix.com]

Thanks to everyone for the suggestions!

If you are somewhere where it gets cold you will kill the Sonos after a few years. When it gets cold and the device is warm you get condensation on the electronics, that will eventually corrode. I live in the northeast and I just can't put indoor equipment outside, even if it is protected from precipitation. I have had amps in unheated changing rooms, after a few years you can see the discoloration where the condensation collects near the warm parts.