When I was at the Dahlia Society of Ohio's member show in October, I saw several of the exhibitors carrying buckets that had been modified to transport dahlias. Their exhibits arrived with perfect foliage, and petals undamaged from transport,
The method is simple: find a low bucket, drill some holes in the sides above where you want your water level to be, attach stakes or rods, and clip your dahlias to these supports with plastic hair clips padded with a little cotton or other soft material.
Blooms are set far enough apart so that their petals don't become damaged
The stakes can be bamboo, wooden dowels, or fiberglass
Secured on top with a padded clip to keep the heavy flower head from snapping or weakening the stem.