I decided I'd better carry a spare kill switch magnet cap in case the worst happens at an inopportune time.
$25 for a new one, or fire up the CAD, borrow a mate's 3D printer and bingo.

If anyone cares to print more it is available on Thingiverse:

Magnetic Stop / Kill switch top Leonelli Apico Jitsie

I tweaked the design a bit so a bootlace fits through the hole, avoiding the tiddly bit of string.
Also included a bead to tighten the lanyard on your wrist.
Common 10 x 5mm disc magnet (approx $1) slips in and just melt the lip to hold it.

I found melting the surfaces of the lanyard and squashing them together made a solid & neat join.

@biffsgasgas made up some spare magnets for us to use, they are called "Nancy's"

Kids modelling clay (a couple of £pounds) wrapped over  magnet and left to go solid, no need for CAD or 3D printers

Edited February 12, 20188 yr by onefootup

I have made a silicone mold of the magnets. As @zippy mentioned i mold them up with some carbon fiber and attach a bra strap to them. If  you lose or forget yours then you ride with Nancy all day. I need to make a female version with some tighty whitey elastic and we will call it Dirk or Yanni or something. Team WGASA is an equal opportunity organization. 

--Biff

Made mine from a nylon rod turned on lathe. The magnet was pressed after