I don't know if the following suggestions work for you. For the PDMS sheet, you can eithor spincoat a wafer/plate/something like a plain or make a mold for injection molding with your desired depth. You need to try the time and the spin speed for spincoating.
We have used Chloroform for diluting PDMS to make thin layer of it on slide glass. Please check Surface Treatment section of this paper: https://www.sciencedirect.com/science/article/abs/pii/S0925400519316119