When you say you may add a half inch ring, it sounds like you already have the box? If so, and you do not want to sell those subs and go with a different size driver, then some sort of spacer or another baffle to mount over the hole and the sub to that is about your only option.
If you do not have the box, and do not want to build it yourself, then why wouldn't you look around for another box? lol
And possibility 3, if that box already meets your maximum dimensions, putting a spacer to make the sub stick farther from the box may cause the surround or and other part of the woofer to rub on the floor while playing, and could lead to mechanical failure.
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