It's best to have some way to bond the slates together. Bondo will fill the gap, but not create a bond. If the slates ever shift, you will have a visible seam.
I use drywall tape (a tip from Jay Spielberg, A-1 Billiards). You can layer it as thick as you need, and it wicks glue quite nicely. In your case, the slate is touching at the bottom, but not the top. Therefor, you will need to stagger the depth of each layer. The bonding area does not need to be full-depth, but it helps, to ensure a solid bond.
Since your paper will be quite thick, it is easiest to use a knife to slice the paper along the edge of the slate. Cut it from both sides, and it should fold over and pull off. Use a utility blade to scrape the glue/paper flush with the slate. Then fill the seam with Bondo, as per usual.