The original flute and box went up for sale a while back, and the dimensions given for the box were 12½x3½x3½in. Note that auction dimensions like these vary in their accuracy from listing to listing, so take them with a grain of salt as it's entirely possible they are rounded off by +- 1/8 or 1/4 inch or so. But they give a good general idea.
If the 12.5" box length in the auction is accurate, that means the original flute was about 10.125" (~257 mm) based on screen caps.
I'd say your box seems more or less proportionally appropriate for the length of your penny whistle - both flute and box will be larger overall but they'll look in scale to each other!
Thank you, E!
I was lying in bed last night imagining how I could lengthen my box. I figured out a way. But it would have added another day to my build.
As for the packing foam, all I can find in my area is white. Walmart has the foam that can be used to stuff seat cushions but only in white. Luckily Google just gave me this video on how to make it a different color: https://www.youtube.com/watch?v=hihfQgoeHPg I am thinking a light to medium grey from all of the reference photos.
So today I am re-attaching the hinges, making a magnetic latch, squaring up the box bottom and lid, and making the leaf pattern on the box surface. Work will continue on the flute of course. I am hoping to have this finished by the weekend.
And I found this for the pennywhistle: The Inner Light - STAR TREK - Tin Whistle Tabs Play Along Tutorial