So, I've never made a mold before and I've never made a casting. I've been reading the Alumilite online examples and I went ahead and bought their starter kit. I figured if they can tell me how to do it online, they can have some of my money.

The first thing I did was use modeling clay to plug the openings:

I'm going to have a two part mold and I won't be able to have those openings in there. Note that I really should make sure to smooth things down on the prongs, but hey, I want to know where to cut those holes back in. I'm banking on having a faint outline there to guide me.

You can see things are pretty smooth still though:

Note that the surface I care most about is that screw connector shown at the top in that picture. I really want it to be level such that the fan will not wobble. But given the way I'm building the bottom of the mold, this will not matter that much:

I've elected to have the seam be on the base and I've pressed the retainer into the clay. At this point I need to clean up the top of towers (look in the lower left one) or I'll be cleaning up each and every cast. Another decision is whether I want to add some more clay to provide material to shave off for the base connections. I.e., I pretty well near went into the outer wall on my earlier attempt. I still plan to add some material around the feet when I prep the bottom half.

Oh well, I still need to figure out what I am going to do about the casting box, so I've got some time.

I'm also hoping the resulting cast will be strong enough. The demos produced car model parts and I think that will be sufficient. I'll have to see - I may need to get different casting resin.