Agree with Boaz, just solder a blob on top of the battery and be done. Never even used extra flux, just rosin core solder wire does the job: load the solder iron with a bit of solder, pre-heat the battery top with that for a few seconds, feed extra solder wire until the blob is to your liking, done.
I did go the magnet route but mainly because it’s the easiest to undo if ever I wanted …… they arrived today and an initial test worked a treat … I haven’t tried wrapping them in foil / hot gluing the corners yet but will certainly give it a try!!!
The ones I bought were 4 x 1.5mm they possibly could be a bit bigger diameter and as they’re so cheap I may order a couple of other sizes any way as useful to have to hand.
Witterings, the reason behind wrapping the magnets in foil is, as usual to me, to increase efficiency. I've done my math and, even at 30A (!) of current flow, the amount of equivalent wire section on those magnets is good enough to barely drop 1'1ish millivolts (less than 38μΩ of equivalent resistance). Neodymium is not a very good conductor of electricity… 
djozz & Boaz, your successes in wetting those positive battery tops with solder and without perceptible cell damage lie in the fact that they are shallow metal domes, allowing you to heat them fast without significant enough heat spreading to the body of the battery. Now, go try that on those solid negative pole battery butts… 
My technique also allows to efficiently interconnect high discharge rate battery packs with ease: a good roll of copper sheet, magnets & a powerful soldering iron.
Cheers ^:)
Why would you need a blob of solder on the negative/ bottom of the battery? I’ve bought - well more torches than I would ever need since finding blf - but have never found one that won’t work with a flat negative bottom cell
I prefer the solder method for the positive, but I do like your way of wrapping and glueing the magnetic, I’ve used magnets but stopped when I had one shatter, luckily it didn’t short the cell, but your wrapping method would keep it nicely in one place if it does shatter