Portable? They are normally linked into rain gutters. Once full, they are not portable. When empty you may move them around at will. Curious what you have in mind? Where do you live that you get 6" in 6 hrs.? If that is the normal precipitation, your rain barrels will overflow regularly. But you said infrequently. In any case, a typical arrangement is the barrels remain in place to collect the water. Then later you use it as needed.

Capturing rain water in a Mediterranean climate requires a lot more than barrels to be useful. 6” of rain on say a 1000sqft roof would amount to 3600 gallons of water. A 55 gal barrel will fill very quickly (in minutes) and will last maybe a week into the dry season that lasts 8 months. Also, a 55 gallon barrel will weight 440# when filled with water making it not at all portable.