I looked at every single beverage bottle in every grocery store imaginable before deciding that Gatorade bottles were the most interesting and were the best size. I cut a hole in the bottom of a regular old plastic cup, and it fit perfectly over the lip of the bottle. A coat or two of a combined spray paint with primer in grey made it look perfectly futuristic.
Try as I may, given all the bumps and ridges, I simply could not figure out a way to glue or strap these two bottles together. I wound up taking 2 small pieces of scrap wood and screwing them together in a "T" shape, and spray painting them to match. This way I was able to get enough surface area to crazy glue the bottles to, and it worked like a charm. For the straps, I used adjustable nylon straps from some tote bags we never used. I stapled them down at 1st, but the wood was very soft and wouldn't hold the staples, so I secured the straps with screws.
The flames could not have been easier! I just took craft felt in red, orange, and yellow, and cut out wiggly shapes. (The piece of orange I had wasn't quite long enough, so I sewed it a little farther down on the other strips so the ends would all be even.) I then tied two bunches together with rubber bands, and stuck them through the holes in the cups, up into the Gatorade bottles. They were thick enough that they didn't need to be secured any other way.
And with that, we were ready to count down to blast off!
I love how our jet pack turned out, and I hope this post helps someone out there with a little one who wants to be an astronaut this Halloween. :)