What ever carrier you get, place suitcases in yard heavy duty bags! We had a soft one and got all the way into Fl and hit HEAVY rain. Everything in the first suitcase was saturated.
If you are getting a van, get a roof rack with crossbar latches. You can invest in a carrier, Thule is a top name. But it is an investment. For example the cross bars for our Oddysee van were about $180.00.
The soft side carrier we had fit three suit cases, it was about $65. We put a soft packing blanket under it.
What worked better. Was the hitch with a tailgate carrier. Same thing, wrap everything up in heavy plastic. I am not sure but by now they may make a hard shell carrier for a slide in hitch. There is less drag and less gas expense.
You may find a used one on Craigslist, or an auto finder sales paper.