well the front ones and back ones are done differently, the front you are correct, i did find a coupler and used two normal bolts, i just made sure its tight, and its held up so far this season, and hasn't come loose at all....,
the rear ones i actually used a threaded rod, and two nuts, it doesn't look as clean, but i didn't find any bolts long enough for the rear axle.....
however i'm not sure if its just the wheels i have, or just me being lazy and not keeping my bike shiney and clean. but the rear wheels have turned somewhat yellow from all the road grime, and whatever else it gets in contact with.....