I hope the more experienced chime in on this....
In my much less experience opinion, I am afraid that neither would work well. Plastic will obscure or alter the texture fro your slate to much and I think that cooking spray would not provide adequate release from the porous surface of the slate. Just my opinion and I could be completely wrong, but what I have been doing requires that the mix stick well to other surfaces and I have had little trouble getting it to do so even when the surface was slick or artificial (like a smooth styrofoam). I have, however, had the misfortune of it sticking to some of the natural rocks in the area I am working in.

The DIY show I was just watching showed using a thick wax material in a can. Looked like a MinWax yellow can. Maybe that will be your trick. I am even newer (just found out about this today) at this so hopefully some of the more experienced persons will chime in here soon.