TubeNet

I am shipping a tuba without case or bag to Barcelona.
My plan is to build a box out of 0.24 inch plywood with about 4 inches oversize.
Then wrap the tuba in a serious amount of household film, let the tuba rest on pieces of polystyrene in the box and then fill up the empty space with expanding PU building foam through holes in the box.

How does that sound?

One of the best posts on TubeNet was by Norm Pearson about 10 years ago. Thankfully it's still here.

Packing a tuba for shipping (lots of pictures):

If you decide to use foam, use nonexpanding foam, the type for doors and windows. Expanding foam will find it's way into everything. and may put enough unwanted pressure on the horn and box. I learned the hard way while building my home.


CCC

Better think about how you will remove the foam when it's time to unpack it in Barcelona. Try it first with a junk euphonium.
I once tried to make a bell insert out of foam from a can. I first lined the inside of the bell with plastic sheeting and put a plug at the bottom. It didn't work out well. The foam that is not exposed to air did not dry or expand, but remained as a sticky goo. Some of the goo found its way past the sheeting and into contact with the bell, and was not easy to remove.

I would do as you say but...

--Replace the expanding foam with packing peanuts.
--Replace the 1/4" plywood with one closer to 1/2"

Here are two more suggestions:

http://www.galvanizedjazz.com/tuba/BigConn.html

http://galvanizedjazz.com/tuba/4sale/KingCrate.html