I made the base for the arena walls and bleachers out of proper blue XPS foam, more solid than the the green stuff I used for the arena floor. The Editor's box is made of 5mm foamcore and houses the portae into the arena proper. The arena wall is just some heavy cardstock, cut to size and bent into shape. It served not only to help hide all the sins but to give a nice smooth finish. I was testing the look of the arena here - seeing if it would look grand enough.
The bleachers were made out of white packing foam as they would not see any play and would be covered in as many spectators as I could manage. I used a large oval template as a guide and just kept removing about 1.5cm so the template shrunk each level. Stairs were cut into the bleachers to make the arena as realistic as possible.
Here you can see all the layers of the bleachers from the outside wall.
2024-05-14 Your project has been visited by the unofficial Hobby Hangout. Huzza!