A friend of mine needed a replacement key shell for the key of his Honda Prelude.

I don't quite know what the material of the original was, as it was squishy and sticky by now.
Could have been any kind of plastic that just disintegrated over the years.

First I printed it in one piece out of TPU. The print quality wasn't great and it was quite a struggle getting the key inside it.

I finally printed it in two parts out of PETG-CF, glued them together with the key inside and melted some filament into the seam with a soldering iron.
I then sanded away the access at the seams and that's it.

Best printed tilted 45 degrees on one of the lower “edges”.
Just separate the two parts a bit and paint manual supports on everything with 35°, except the text and lines on the front.