Mold it up
1. Prep the part The other half of the box can be built right on-top of the blocks that made the first halve's mold box.
2. Build a box
3. Bury the Body
4. Mix some RTV
5. Let the Air out
6. Drown the Part
7. World turned upside down
8. Drown it again! The other half of the box can be built right on-top of the blocks that made the first halve's mold box.  The vacuum had got low enough to evacuate the air under these mounting pieces, drawing the rubber in.
9. Fossilization? Give me resin!