Shim kit offered from your same vendor:
https://mercedessource.com/store/var...him-washer-kit
Many well-stocked local hardware stores have rubber/plastic caps in various sizes. IIRC, the gray ones (they seem to be size/color coded) are the correct size for the injector tops. OTOH, I never seem to have the time to get those, so just use pieces of aluminum foil to cover them, pinched around the threads.