I bought something like this:
GROSSē Folding Wardrobe Organisers Cubes Storage for clothes, kids toys, Bathroom Plastic Organizer Closet Shelves Under Stairs Shoe Rack Space Saving Solution, White: Amazon.co.uk: Kitchen & Home
I joined all the panels in one long line with zip ties. I collected cardboard from parcels to lay over my carpet to protect it. You may meet to put heavy things like books against the sides of the pen as your hamster will probably be able to push them.
Good luck!