First, remove all the working parts of the subwoofer.
Most commercial boxes are only glued at the edges - it's cheaper if you use less glue. Try a hot air gun on a seam, with a thin taping knife to get underneath. If it starts coming away, continue. If it makes no difference, then you're going to either be cutting it off with a knife, or breaking the box down so you can use it to make templates for a new box that you build yourself.
You can also go on Amazon and find a pre-built, unfinished enclosure that will fit your car and the drivers that you've removed from this one