Recessed Reed Switch

[chris_j_hunter]

spec' says up to 15mm ...

[achapman]

I dealt with the problem of having large gaps in the following way. On the door itself (the moving part) I stuck a flat magnet and on the frame I used the reed part of the reed switch (the bit with the wires). The magnets in most reed switches are fairly feeble and only work over a short gap, so by using a more powerful rare-earth magnet (e.g. https://www.first4magnets.com/rectan...2kg-pull-p3279 ) you can use them with a wider gap.
It is also easier to fix them; no drilling a hole in the door, just stick the magnet on the surface.

[cliffwright]

I can't seem to find my original order so not 100% sure where I actually got them from (My usual haunts are eBay or Amazon ... ) but I use something very similar to these;

https://www.amazon.co.uk/sourcingmap...ntact+Switches

They're pretty small and discrete, and you can fill / paint over them if you really wished.