I filled the gaps with Gorilla Glue construction epoxy and let it cure overnight.
And coated the inside surfaces with more rubber sealant to add to it's strength. That stuff does not like to be applied to wood. Didn't actually brush any of it on, more like smearing it into place. I think it's cause as I was applying it, it was also being absorbed a little bit. I'd like to think that'll add to its strength that much more.
Going with a sealed box as it takes up less space and enjoy how they sound.