I'm with Thermond on this one. Latex rubber tubing works wonderfully. I use the "Theraband" blue exercise tubing -- not the cheapest, but reliable and a foot of tube will give you ~50-60 bands.
Personally, I don't trust the O-Rings included in the kits. They just don't hold tight enough for my taste. Sure, they go on easier, but if it's easy to put on, then it's not holding the prop firmly against the adaptor. If it's not firm against the adaptor, it's going to wobble, fly poorly and get loose.
Might just be my paranoia speaking but make sure it's really held on TIGHT!