So to expand on what I said above, I learned that, despite the very viscous nature of the new mold material I used here, the stuff actually managed to become more flowable to the extent that it actually oozed through the seams of the Lego bricks which I thought would be tight enough to prevent this from happening. This, combined with the fact that some of the mold material also flowed over the nacelle wall into the inside surface (the material was removed for the pic below) caused the level of the mold compound to drop below the edge of the nacelle, which would be a problem for the piece I want to cast.
The nice thing about the material I used earlier was that it set so quickly that it had no chance to flow anywhere and I didn't need to be as worried about it flowing all over the place. The bad thing, of course, was that it shrank too much when it dried. In hindsight, I should have glued some fences to the inside of the nacelle to allow the mold compound to be poured deeper. In any event, I had to deal with what I now had and I decided to build up the level with the clay I bought:
Here is the mold with the model removed. There's no sign of shrinkage this time though there are some bubbles that formed along the bottom so I expect my casting will have warts all over it that will need to be sanded off.
For added insurance, I thought that I'd also cast a second piece, using Terry's suggested use of clay. For this method, I had to settle on making just the half nacelle because the air scoop would be destroyed when I peeled the model out of the soft material.
After I took the above picture, I had to start over as the clay was sticking so hard that there was no way of removing the mold without severely deforming it. I reset it and decided to leave it for while to cool down, thinking that this would help. It didn't. So I started again and this time coated the model with some lubricating oil. This time, I was able to remove the model.
So now we have the resin mixed and added to both forms and we now wait again for a couple of days to see what's up.
If this doesn't work, there will need to be a plan D!