Like the others I think the clamshell on top is so large it becomes a distraction. I'd look for something that specifically links with pet. For example, if you have the license tag, perhaps make a wood holder for the tag. Otherwise, not sure you need any kind of topper. Just bring the "wedding cake" to a nice conclusion with one more smaller step. If you feel like some kind of topper is preferred, I like Tim's suggestion because they resemble crosses, a symbol typically associated with a memorial box.