According to https://latexcrafters.livejournal.com/188596.html
Use a thick piece of plate glass, carefully leveled… pour thin runny latex on the table, preferably, vibrate the table to flatten the latex and work bubbles out. the latex should ALWAYS be poured through a strainer (I use paint strainers as they are cheap and effective).
Liquid latex (even thinned) will tend to adhere to almost anything. Glass and other highly polished surfaces less that most. For best release, treat the surface just before use with a good latex release agent (typically a solution of calcium nitrate or calcium chloride) and let dry, then apply the thinned latex milk solution.
Heat is less important (since most liquid latex that would be used for this is self-vulcanizing) than constant airflow for drying it quickly. A low-speed fan blowing ABOVE the surface (not on it) will help the latex cure more quickly.
The biggest issue with most of the techniques here involve surfaces that are unlikely to be perfectly flat. Without an industrial planer, the thickness is liable to be uneven throughout the cast latex sheet. Such ‘custom cast’ sheeting is best used for detailing rather than primary construction materials.