You will want to do a stone mold for this. Make a box around your sculpture, make sure it is sealed and then pour some kind of plaster over it, whether it be ultracal or hydrocal. Pour in your latex and then wait for it to dry.
Step 1. Make or use a container you can put your sculpt in, and ensure surface is flat. And also make sure you can take the container apart so you can get the piece out when it's set.
Step 2. Secure sculpt to base with clay. Make sure there is gap for the plaster to bleed under the sculpt though.
Step 3. Apply a release agent to the sculpt, vaseline works well for quickness and it's cheap. You can get a spray if you want.
Step 4. Mix your plaster and pour into the container, start at the lowest point and allow to build up and eventually cover your sculpt. Tap the box a few times to get any air bubbles to the surface. Leave to cure, depending what plaster you use just leave it over night so it's fully set.
Step 5. Remove plaster from the container and the sculpture. Clean up the plaster from any vaseline or release agent with a soft cloth and warm water. Careful not to scrape too hard and chip away details.
Step 6. Pour latex into the plaster mold, tap it again after pouring to get rid of any airbubbles and allow to set. Remove when it's fully cured.