Unable to build an image for Raspberry 3, with custom gadget



i need to create an Ubuntu Core image for a Raspberry 3, to use a kiosk, when i need to rotate the monitor and set a custom splash screen.

Cloned the https://github.com/ogra1/pi-kiosk-gadget and compiled with the new logo and the rotate command.

Then i tried to create the image with this model:

          "type": "model",
          "authority-id": "",
          "brand-id": "",
          "series": "16",
          "model": "my-pi3",
          "architecture": "armhf",
          "base": "core18",
          "gadget": "pi-kiosk",
          "kernel": "pi-kernel=18-pi3",
          "timestamp": "2019-09-13T11:08:07+00:00"

and the built the image with:
sudo ubuntu-image --snap pi-kiosk_18-0.1_armhf.snap -O pi3.img pi3.model

The resulting image is about 500MB but when i transfer it to the SD, while the “boot” directory contains all the files (including my custom commands) the “writable” directory contains only the “lost&found” and “lib” directories, and the image doesn’t boot.

What i’m missing?