I've been playing around with this project off and on for a little while and am now feeling pretty pleased with myself:

 

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3D printed adapter for PDL 600 style two-part light switches. Clips on to the existing indentations, and is deep enough to cover the switch so no more sticky tape or labels with "do not switch off" required. If you do ever need hard control of the light, it pops off with only a bit of effort (depending on how much paint you have on the switch base underneath).

 

As per the previous pic, I can stick on other controllers (Xiaomi / Lumi / Aqara Zigbee button and sensor in the image above), or a bracket to hold a remote:

 

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That one is a Yeelight bluetooth remote.

 

Or just use it as a blank if you use your phone, or have a tablet/PC stuck to the wall (Mrs MDF has proved surprisingly tolerant of this approach):

 

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STL is here if anyone wants it. I printed on an Ender 3 Pro, Creality white filament, Cura + Chep Magic 2.0 profile, no supports, no infill. Takes a little under 3 hours and uses about 7m of filament.

 

I'm also playing around with an open version for full face plate controllers like the Hue dimmer switch. This prints substantially faster and uses much less filament, but I don't have a Hue dimmer switch to test. If anyone wants to flick me a pic of the back of a Hue dimmer switch along with some reasonably accurate dimensions I can check.

 

EDIT. Holy huge images batman. Edit to use the thumbnail link.