Redesigned the work of @LayerCake in Siemens NX (academic version). 

Redesigned the spring clip as my printed results weren't holding on the rail sufficiently. The leaf spring system is now longer and clamps better, so the module can still slide, but can no longer be pulled of as easily. It is angled to the model to supply the spring force. 

As with the original model, the Shelly modules are held in place by spring tabs and the clip walls. 

Also added models for RGBW2 and Plus 1 addon. 

The RGBW2 is slightly larger but flatter than the Plus 1. 

The Plus 1 addon model provides space for the extra mounting clips of the addon, is taller and provides a bridge where both modules meet. The extra connections are still accessible. This bridge has an integrated support which can be broken off after printing. 

The Shelly I4 DC also fits the Plus 1 clip, and can be combined with the addon in the Plus 1 addon clip. 

Printing advice: see included 3mf files, but:

• ABS or ASA preferred, PETG if required
• 0.15mm layer height, 0.4mm nozzle (relatively small parts)
• Sufficient top & bottom layers. There shouldn't be any infill in the leaf spring. 
• Brim is advisable, as the bottom contact area isn't too large and the part has sharp(ish) corners
• Infill type shouldn't affect the part too much

I printed in Azurefilm ASA white on Sovol SV06+. This worked without an enclosure. However, some of the brim stuck very well to the print bed.