Man I don’t think there’s anything besides what you already mentioned (the $35 seam seal) that I could recommend. It’s either you do it right now, or you do it again later. I did however, use some contractor grade liquid nails or something on my first jeep I did floors on. I also used self tappers vs rivets because I was an idiot. So in theory, you could use liquid nails like I did with rivets like I didn’t and it would probably be fine, but it’s just not the right way to do it. Gm seam seal is really the best way to do it. I just did a driver floor patch on my current XJ and used rivets and seam seal. And I used license plates. It’s the best way without welding to do it. Good luck with whatever you choose tho. You gotta do what you gotta do when necessary.