AS long as the plug is airtight, it will work as a sealed enclosure.
A small air leak in a sealed enclosure will yield poor performance however, so make damned sure the box is airtight before you get bumpin' the Lambda.
The best way to determine if the box is airtight, is to simply push on the cone. If it pops back up immediately like a spring, you're set. If it hisses out air and slowly rises back up, time to start looking for the source
Make sure you buy a foam gasket to place between the sub and box. If you don't air will leak out between the rubber edge on the Lambda and the box. Don't just bolt the sub down over the carpet. You can get the gaskets pretty much anywhere that sells 15" subs.
