Im looking for some ways or adivse. What's the best way to bolt down my seats in my tunnel? it looks likes previous owner kept removing original seats and re bolting them or if they kept coming off on its own. I have I guess what it looks like,  half a stringer? So like a 1x1? Looks like where the bolts used be got rotten. Not sure. I'm a newbie and not to familiar with fiberglass. For the meantime I put some silicone inside the holes to prevent more water getting in. I was thinking about refilling it some resin? Note, the rotten wood you see in the first pic is from the original seats that where in it. It's not from stringers
2nd pic is what I was thinking of doing but like I said my stringers won't allow me to do so.

Would something like this work? Maybe I can put these in and epoxy them over and just screw seats down onto these fittings?

  I would glass in a 3/4" marine grade plywood tied to both off your runners.  Then I would attach seats to the new well glassed plywood.