You can successfully build your own amplifier isolation box (as outlined here in an older post), but now you can also buy one pre-made from a manufacture who already makes guitar cabinets. Randall Amplifiers makes not one, but two different models.
As with all isolation cabinets, the sound isn't totally quiet as there's still some leakage, but it's decreased a great deal. Also keep in mind that iso cabs do change the sound. You're recording in a box so it tends to sound a little boxy as a result. That said, the tradeoff for a lot less leakage and neighbor complaints might be well worth it.
The Randall ISO12C retails for $399 and the Isolation 412 for $599. Keep in mind that you still have to supply the mic and holder. You can find more information on the Randall site.
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