The other option you have is to use the cave as the sub's hibernating spot if you don't like the way it sounds in there. What I mean is that maybe to keep the wife happy or whatever it stays in the cave most of the day. Then when it's movie time you can slide the sub out and place it where ever it sounds best & enjoy the show. Show's over and sub goes back to sleep in its cave.
I'm not sure if I heard correctly but this is for an EP350 right? And someone else pointed out that the 500 is the same size, is that right? If so this is a good thing because if you're building a spot for it I'd hate to see you outgrow the 350 within a year or so & want a bigger sub but be stuck with the 350 because nothing else would fit into it's spot.
But if you can safely upgrade to an EP500 and still fit it in the same hole, then that's a plus.