What I like to do is get a piece of glass or thick clear plastic, mold release it!! I like the Stoner release for this application, as I think the UMR hazes the surface a little more than the Stoner. Only use a little to reduce witness marks for the release but make sure you have enough to release it. You would then over pour the cavity, and use the glass to slowly squish the excess material (from one side to the other) to create perfectly flat back. You do not want to squish the glass past flat. We only want to lay it on the back side until it is flat. If you force the plate too far down, when the mold retracts it will suck in air. So lay the glass on from one side to the other in a slow smooth motion and place an adequate amount of weight on top to hold it in place until it cures.