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To mount the Wii/Wii U sensor bar in a Besta cabinet, I’ve created a simple mounting bracket.

3D printing two of these, the second a mirror of the first, the sensor bar is a friction-fit. It slots in to the cut-out section on the end of the bracket.

In addition, there is room inside to thread the sensor bar cable, should you wish to run the cable along the side of the cabinet.
It’s advised to print the bracket with the screw side flat on the bed (essentially like a tower), to ensure there is enough clearance inside to pass the end of the cable through. While it should be a small enough gap to bridge without internal supports, any sagging will obstruct the cable.

It’s designed to sit at the top of the cabinet, with screw mounts on either side to attach it to the top shelf screw holes. Adding a strip of double-sided tape to the top of the bracket is recommended, to hold it level.
It’s not the most sturdy of designs, but given what it’s used for, it’s more than enough. I’ve had this fitted for over 3 years, until I removed it to take photos for this post.
If you want to print one yourself, you can find the files over on GitHub.
Wii Sensor Bar Bracket for an Ikea Besta Cabinet
Files to print a Wii sensor bar bracket for an Ikea Besta cabinet.