there are inserts that fit into the holes in the head at each plug that rise above the head like a cup, and then the tube slides into the insert. i have them on mine, and they work great. keep at least 90% of the oil out. i got mine from a guy on ebay, but it was a while ago. just try a search on ebay with variations on "426 hemi spark plug tubes" and you should find them.

or i would assume someone like mancini would have them, there are a number of people out there who make them.