+xVery interesting process and nice end result.
To limit the use of silicone , you can build a former for the reverse that fills a lot of the dead space that would normally fill with silicone. You can use wood or foam or even polystyrene cut to shape. Suspend it above the void leaving a 1-2cm gap to be filled with silicone. Then do your pour. Once cured you can either remove the former or leave it in place for support. If the material you used is not porous, then you can remove and refit at will so you can refit it to support the silicone during casting, then remove to help flex the silicone for demoulding.
Thanks so much for your quick response Warren.