I had this same problem with a model I was building one time as a kid, so I threw a few fishing weights in the nose cone and to hold 'em in I filled the thing full of the same model glue I used to assemble the jet.
Turns out that wasn't such a hot idea, as the glue just kind of melted the nose cone and made it all limp and useless. Kind of like DejaVu.
SOB