This morning I removed the plaster casting from the mold. It came out quite easily using the embedded bolts as finger grips.

The plaster has to dry until it feels dry to the touch before prepping it further. Then I will remove the bolts, sand the base flat and dress the edges. I'll also need to fill a couple of bubbles that appear to be the result of my not having mixed enough slurry on the first pass. (I knew what the volume of the canopy was from having filled it with water during the making of the silicone mold. What I forgot is how many ounces there are in a cup when I was mixing the plaster. So I didn't mix enough and had to add a second layer. Rookie mistake!)

Overall, the plug looks good and should be ready shortly to make some canopy's.