There is only one large costume prop we've made to-date, but more will be added as they are created.
Ghostbuster Proton Pack
We built this scaled-down proton pack to fit our 8-year old, back in 2011. It's mainly constructed of lightweight materials (foamcore) and a lot of miscellaneous items including a pie tin, corks, pill bottles, washers, a plastic electrical box, IDE cable, toothbrush holder, and other little parts, hoses, and wires. It's all mounted to a thin wood backboard with parts of a cut up backpack bolted to it.
Although there are no actual electronics in it (no battery-powered lights or sounds, sadly), I did build a few parts so that we could insert glow sticks to make it light up. You can see this a little in the image below, but it was much more visible at night.
Harry Potter Broom
This was a Nimbus 2000-like broom we made to go with G's Harry Potter costume for Halloween back in 2009. It's just a single piece of shaped wood with a few coats of polyurethane, straw, and gold-toned heavy gauge wire.