Raspii--if you do a very diligent search on the forum, there is a post that shows how to cast your own silicone plug boots. I don't have it marked, but perhaps someone else can help you find it.---Brian
I used "soft vacuum elbows" from a crowd called Dorman. Pt number is 47410 [google will find them] They are just the right profile and will take a 3.5mm diam. silicon cable. Just trim the end to suit the seating of a 1/4" x 32 spark plug.
They fit in well with my Howell V4.