I bought a nice sealed AGM battery for another purpose last summer (separate story). It's a RoadRunner ETX15L. It's taller than the GS500 battery (6.56 inch versus 5.75 inch). My GS battery was struggling this past winter, so I used the ETX15L the GS... worked great (14Ah, sealed), but it was too tall, and the GS500 seat wouldn't clip on. To get by, I used a cargo net to hold the seat onto the bike.
This weekend, I cut the bottom piece off the battery box, so now the battery sits on the frame. That wasn't quite enough room, so I cut the foam off the bottom of the seat pan, where it rests on the battery. Using the side-mount terminals on the battery, that gives just enough room to snap the seat down.
But I think I'd suggest just buying the right size battery :o