Snap-On has a socket with a magnet built in in 12MM shallow. Mine has installed over 250 knock sensors and looks like new.
Their swivel impacts are a must have. I have 10, 12, 14, 17 and have warrantied them 5 times on the small ones and 4 on the big ones over the past 5 years and they are damn durable. The number of times warrantied isn't a mark of the quality, just how hard I am on them.