We did have to go buy some tie-downs so that we could strap the cart to the roof of the minivan, and it took a while to work out a way to tie it down securely, but Lisa figured it out. We took the van out for a brief test drive on the freeway, and there wasn't a trace of rattle from the roof rack. We then refueled the Astro so we'd be all set to leave on Tuesday afternoon for Bishop (leg one of our trip to Westercon) and headed home.
As we headed home, a thunderstorm rolled in. While the cooler air was welcome, the new cart is plain wood (there hasn't been time to paint it), so Lisa got a tarp and tossed it over the rack to keep the rain off overnight. We don't expect rain the rest of the trip.
Now I just need to remember not to go into parking garages until we take the cart off the rack.