How to Mold and Cast a small thin piece of plastic?

[Ann N]

I am building miniature furniture (dining chair) and have created one piece using polymer clay. I want to create identical pieces, for a dining set, but I feel like using the typical molding putty would damage the original piece, as its very thinly baked clay (its already a little cracked!)

So how do I create a mold of the original, e.g. what materials should I use?
And what kind of resin should it be casted in?

I'm a hobbyist so I can't afford the injection molding stuff..