WarmGlass.com

I want to create a SLIT between base and topper, the slit size will vary
from 2-3 '' or 4-8"Length X 1/8th to 1/4" depth into the fused item. The rest I hope would fuse together except my Slit area.

Has anyone done this, if so what did you use to keep it from closing up?
Or does anyone have any suggestions on how to proceed with my idea?

Thanks
Camaro

If I'm understanding what you want to do right, I would try fiber paper. I've seen it done once - The artist felt it created depth in the piece.

I've used fiber paper to create a small opening for a frame. It did leave a rippled sort of look on the one side from the fiber paper.

I've used fiber paper, fiber board, and kiln furniture wrapped in paper to create voids. You'll always have some edge irregularity, but with any of my projects it's been acceptable.