This is a premade mould based on Rozzo's Happy Ditto, optimized a little for more consistent demoulding when using soft silicone.

Prints without supports with the "bottom" of the Ditto upwards. I used 2 wall layers, 1 bottom and 3 top layers with 10% infill, although larger versions might need more infill for support.

For the silicone, I used a 1:1 silicone mix from a "squishy kit". Post-curing it may have a tacky surface, so I dusted with a tiny amount of cornstarch and rubbed it off with a microfibre cloth.