First, go to the very helpful and detailed Life in a Typewriter Shop post by Tom Furrier on how to remove & reinstall the platen from the Hermes. Read his intructions very closely - it can be an easy removal but you need to be careful.
|Remove the platen as per Tom's instructions.|
Next: find an inner tube that roughly matches the diameter of the platen. I went to a few local bike shops and they were more than happy to give a bundle of used tubes from their workshop. Honestly, they replace many of these every day and the just bin them. Some of them will be fat and unusable (mountain bike tubes) and some just too narrow (the skinny race bikes).