Man, that brings back memories. My folks owned a resort on Lake Havasu when I was a kid, and we used to cast scorpions, wasps, tarantulas, any poor critter that was too slow to avoid capture.
We were making bolo tie clasps, paperweights, and stuff like that that, but the process was the same. Cast the top layer, let it gel, put the specimen in upside-down and pour the middle layer just deep enough to cover it. We would use mounting pins poking through styrofoam boards to hold them down while the second layer cured. Then we would cast the final layer.
Don't mix the resin hot. The bubbles need time to work out from around the bug. You can help them with a needle.