You will need a proper sized magnet(s) and an M4 nut. The nail i used is about 62mm long and 3.2mm in diameter. You need a M4 drill for the ptfe tube hole.I used a 4mm drill as well for the other hole but i wish i had something around 3.5-3.7mm. Eventually i had to make the hole even larger and then use a piece of plastic tube that had the right size. i do not know the origin of that part, its just something i had around but i do not think that you need it. A bit of grease might help as well. And of course a small hall sensor board, look for "hall switch sensor smart arduino" on ebay, i used the black one with the hole. i think it use an LM393 chip.