Requirements for soil sample transportation over long distance
We did a soil sampling expedition around farms in the UK a few years ago.  We took with us some insulated plastic picnic drinks cool boxes, and a lot of pre-frozen camping freezer blocks.  We bagged up the soil samples, and put the bags in the drinks coolers with the frozen blocks.  This worked fine for our 48 hour trips.  If you were going for longer, you could try to arrange access to a rotation of frozen freezer blocks, or buy bags of supermarket ice along the way to top up the drinks coolers.  The idea for us was not to freeze the samples, but just to keep them cold enough that the organismal diversity was not changed too much during the transportation.