You attach it to the bottom of your battery then insert the battery + magnet into the light. Something around 16mm x 2 or 3mm thick should work.
I don’t have a amazon link but how about fasttech? 15*1.9mm Neodymium Magnets 10-Pack $2.28
If you are using the magnet on the positive end of the battery just make sure that the magnet doe not come into contact with the flashlight body itself from a knock or other reason. If the negative and positive are joined together it creates a short with bad results. If you are unsure about anything it maybe safer to use longer cells. Saying that I have used magnets in the past, just carefully. All the best in your quest. 
Awesome thanks for that.
Cheers mate.
Electrical tape should do the trick.
Thanks for the tip.
Nothing beats a solder blob .
i withheld doing them for way too long . You kick yourself for not doing them sooner ..i had way too many lights that were demanding use of my limited longer length cells . i also used magnets for a while and compared to a solder blob they are just a dumb idea .. get lots of flux ... scratch up and clean the tops and ...bip.....1/8th of a second later your problems are all gone . After about 50 you get pretty good at creating the per4fect nub.
The blob
ask for it by name
If you can access the spring(s) it might be possible to stretch them out and lengthen them that way. Especially if it is cheap springs... But be careful so you don't stretch the string too far or even pull it away. I sometimes do this on battery packs / radios where the batteries are a little loose so they rattle or even break the connection when bumped.
A magnet shifted to the side contacting the battery tube is a dead short circuit, big trouble —- any time if it happens on the negative side; if it’s on the positive side then as soon as you click the light on it’s shorted out.
Trouble follows immediately.
Look at any hardware store for nylon spacers — match the inner diameter to your magnet and the outside diameter to the battery tube, and the thickness equal or slightly thinner than the length of the magnet.
Glue the magnet inside the spacer (yeah, finding glue to stick chrome plate to nylon is an exercise for the user, I know … something gooey will work).
Then the magnet can’t slip to the side if you happen to, say, drop the light.
Neodymium magnets and hot melt glue.