How do you deliver in the rain? A fully assembled four tier cake? Do you just have a friend with an umbrella hold it over the cake? In all my years I guess I have had sunshiny events and have never come across this but I think tomorrow night will be the exception!

I would box it. Try moving boxes for odd shapes and heights

A box covered with a clean trash bag.

I have a cakesafe. I don't worry to much about rain.

I put everything in boxes anyway.

Here is an excellent video by Kara Buntin, "acaketoremember" on youtube:

This may seem strange, but whenever I deliver a cake in the rain, I just take a plastic dry-cleaning cover and cover the cake, because it is very light in weight, or I use a trash bag that I cut up the seams to cover the cake. It works.

I learned last weekend to prepare for rain, even if its bright and sunny when I leave the house. It just poured rain. The cake made it with only a few drops on it ( I was not so lucky and was soaked from the waist down.) but since I was a guest at the wedding it drove me EXTRA nuts. No one else noticed and the cake was a gift and loved but still.
Next time I'll include a big garbage bag in my emergency repair kit.

Oh yeah...Remember to tape the big "x" on the box or the flaps can shift, which I found out the hard way.

I use very light gigantic see through trash bags, but i don't do buttercream.