hello
i did this yesterday. i bought a 1/2" bolt, 1?2" nut and washer.
on the rocket, i dremmeled a octogon shaped hole, which the end of the bolt could fit in. i then fiberglassed it in place. on the jet pack body, i drilled a hole, which the rocket, when in place would fit nicely with the screw end of the bolt going through it. on the inside of the pack, i then just put a washer on it and tightened the bolt.
the rocket is very sturdy and can be removed if i need to transport the pack.
hope this helps.
Maybe later i can take some pics.