optionally there are resins for casting that are very strong and can be colored and tinted to look almost exactly like that sub. saves the added trouble of dealing with heat and flow.
a friend bought a rubber mold set to cast chess pieces with, what a nightmare! the molds were of dubious design and the metals we bought from the same outfit were terrible for casting, the finished pieces were way too soft. the pot we used to melt with was a bullet casting pot, it seemed adequate. I eventually made a vibrating box to clamp the molds to, to help with the fill.
the vents in the molds were poorly designed and the sprue lead was too short to allow for shrinkage. the box was simply a wooden box upside down with rubber feet, a motor with a concentric weight on the shaft inside the box to vibrate it, and a block of wood sticking out the top to hold the molds to. I had made a speed controller out of an industrial ceiling fan controller, for a heavy duty die grinder thet only had one speed, it worked well with the box.
I have casted a few lead pieces in silicone, the trick seems to be to design the mold vertically so the weight of the lead forces it into the trickier areas. even the sprue can act as a sort of ballast to the casting and compensate for tunneling from shrinkage.
I even cast a few very low temp metal pieces in urethane to see if it would work. the alloys weren't safe for extended handling. the stuff is made for casting machining samples for quality control and prototyping. a pity since it cast fairly well and the pieces were every bit as hard as the new cast minifigs (the bismuth alloys). the molds bore up fairly well but the urethane did become more flexible while warm.
the mold is just a cavity so it didn't want to fill the tailfins well, so I just ran the metal extra hot (still cooler than regular lead by 100 degrees) and used a toothpick to drag the airbubbles out of the center fin and to scrape the gelling metal into the flat fins. was actually pretty easy to do. if I were going to make a serious mold to do the zeps, it would be a two part mold with the zep nose up and the sprue filling the mold from just above the tip of the nose but molded into the flat half to leave the nose details intact. the tailfins I would cast thicker to make the molds easier to use.
you should look into resins at first and silicone molds, much of what you learn making the molds will serve you if you step up to metals. smooth-on has some good tutorials and FAQ reads, molded in color would work nicely with a design like that. I myself am looking into making some molds to do scarab beetle cartouche/seals to hide the drives and also fossilized trilobites, even got a snappy name, "trilobytes"
just need to get all these other projects done first. if I don't get pocky and darkshines their syringes before halloween, there won't be a flagstone low enough for me to hide under.