Here some photos from a project I did a few months ago:
Unfortunately I didn’t have any camera available in my workshop so there are only pictures of the finished boards. I used plywood for the platform. I created myself a sliding carriage to attach a router to it to mill the form.
To find a wooden semi sphere was a hard task. I could not find any (affordable) one with a diameter > 40mm. I ended up buying a semi sphere off ebay that originally was intended as a coordination toy. It had a diameter of 140mm.