I cant get to the back side part of the hull
Yes , that would be an issue..... !!
If that's the case, and the FG is soft, them an adhesive is the ticket.... As previously discussed here.
I have a small emblem on the rear of my floaty...I put it on with silicone and it has been in 2 seasons now. I lightly scuffed the gelcoat with 800 grit and the applied it.