A mold, I did one time that was supposed to be 2 piece and hollow at the same time was really complicated, since you cannot shrink yourself, go inside the two mold halves and apply the fiberglass, I pretty much applied the gel coat to the two halves while it was open, and once that was semi cured (Soft, but tacky to the touch, and it doesnt lift), I applied about 5 or 6 layers of FG and I bolted the two halves together. The wet fiberglass basically acts like a glue and it stuck together, I then went back in when I pulled the part out, and just secured the parting line with one more strip of fiberglass.
A Jet Pack mold can be just as complicated because it takes at least a weekend to do all of this. Another option would be to have a pour spout on top of the jet pack, where the rocket is inserted, and do a slush casting method with silicon.
Ill let the experts chime in, since my mold was my first of the fully enclosed two part molds.
You can also make a 3 part mold, making it easier to glass the two halves, and the top half and just bolt the top half on.
