Al, I did not build a box behind it, though that would obviously make the sound better. I am actually powering my sub with a five channel Fujitsu amp that also powers the four speakers on my extended pylon. I believe it's 4x45 + 1x100 for the sub. My boat is a '97 Prostar, so it didn't come with a stock system. As for the Clarion grill, no it definitely does not fit. I was thinking the same thing, but it's definitely a no go.
Also, if you're interested in finding a good deal, I bought my sub off of ebay, brand new in the box from bigdogcaraudio.com for $68 plus $20 for shipping. Not too bad...