Transporting a bush -- root ball size & water leakage?

We need to transport a blueberry bush (dig up & wrap in burlap). Two questions:

1. How to estimate the size of the root ball before digging? I've found many articles on estimating for trees based on trunk diameter, but none for bushes. Any tips?
2. If the root ball is large (36"+), any tips for waterproofing around it during transport? Our van will have moving boxes etc. along with the bush and we don't want leakage to damage the other boxes. I haven't had any luck finding a plastic tub or pot with a 36" opening.

Thank you!!