Chess is right, a TPMS metal cap will work and are cheap enough at an auto parts store, but you will have to file it. I would contact Magura
http://www.magura.com/en/service/con...ntactform.html ,get what is supposed to be there, no modding needed. Also check air conditioning caps, they are short and you can get them with o rings in them that rarely leak air out and keep dirt out.
http://www.amazon.com/Santech-Air-Co...9410055&sr=8-7