Usually you would cut a clean, precise hole so the top bobbin can be inserted from the inside of the box. It is usually screwed down to wings that are glued to the neck thru. Sometimes it can be mounted to the box with a spring around the mounting screw between the inside of the box and the bottom bobbin, allowing it to be adjusted for height.
The DownUnder pickup has an oversize top bobbin which is screwed to the top of the box. The hole required doesn't have to be clean or precise, the top covers the hole.
am I looking at it upside down?
these seem cheap enough from china .you can't fall off at this price.
No,you're looking right way up John,the top sits in a hole in the guitar top,the bottom mounts to something inside the guitar,either 'wings' on your neck through or a mounting block or similar,the top doesn't actually touch the soundboard,you just cut a neat hole for the pick up top bobbin to fit into,that way you can raise or lower the pup for best response