On 2017-04-13, Greg Guarino wrote:

I'm guessing I'm a little larger than your granddaughter, but I'm
reasonably persuaded that a 1/2" ply box will be robust enough.

I hafta agree. I wanted to ship a Fender Champ lap steel from CA to
NY:

https://www.well.com/~wellvis/steels/Fender/FenderChamp-StudioDeluxe.jpg

A local shipping store wanted to charge me $80USD fer packaging
(bubble wrap=$60USD! All cardboard!). I made a small box of a single
sheet of 1/2" ply. I bought some foam from our local suplus store,
put it all together with brass wood screws. Cost? $40USD. This
about 12 yrs ago.

Bottom line: I weighed about 260 lbs and could jump up and down on sed
box, all day long, with zero ill effects.

nb