The trivet is about 25cm by 15cm. The pattern helps to hold the hot-baked surface up to minimize heat transfer. The concrete was sealed with three coats of water-based varathane.
The plastic was crudely wrapped in scrap plywood to hold in the concrete as it set. We were a little worried about the casting releasing from the textured plastic, but it was quite easy to do without any sort of mould release.
There were some small gaps in the casting, which we later learned that we could have avoided by painting concrete into the interior details of the mould with a stiff brush before hand. I would like to cast something larger using this texture, so we will try that method out soon.