Bridging cavity before tiling cill

Just had some new double glazing installed in my bathroom. The doub;le glazing is fitted into the outer leaf of my cavity wall. If I want to create a tiled cill, with tiles up to the double glazing, what's the best way to bridge the cavity?

I was thinking of using slates or roofing tiles, and then adding 15-25mm sand and cement, and then tiling straight on top.

two layers of dabbed 12.5mm plasterboard will set as solid as a rock, can be tapped perfectly level and will carry the cavity with ease.

Ah!, good one, Noseall. Thanks