As this project doesn't have a real use case yet, there's no requirement about the housing.

Radio

The original idea was to build the electronics in a Tongboxin C803 radio.

Unfortunately, there's very little room for electronics.  The left side of the housing is taken up by the speaker.  The bottom side is taken up by the 18650-cells.

The LED-segment display is soldered onto the PCB and needs to be de-soldered for taking the electronics from the housing.

Power bank

Another option is to use a power bank housing.  These are fairly cheap, already contain room for some 18650 cells.  The solar panel is an extra, but probably won't be of much use.  The LED-panel on the back might be replaced by some TFT-panel or LCD-panel.  It's transparent anyway.

One of the things that would put me down is the probably low build quality.  There are no screws to hold it all together.

Well the housing has finally arrived.  It's indeed low build quality.  The housing is held together by four miniature screws on either side of the housing.  There's a multitude of issues with this housing, but the main one is that it's not easily possible to mount connectors on the sides (SMA, audio jacks).  There's also no means of fixing the 18650 cells.  You're supposed to weld cells together.  The housing is too low to use an 18650 battery clip.

Extruded aluminium housing

YGKT on AliExpress sells extruded Al housings for a reasonable price.  The metal housing will also serve as a ground plane for the monopole antenna.

I expect to need about 6000mm² PCB area.  Three options remain:

• https://www.aliexpress.com/item/32802694790.html : 88x38x100mm, €8.2
• https://www.aliexpress.com/item/32764144816.html : 88x38x100mm, €6.97
• https://www.aliexpress.com/item/32764024806.html : 63x37x95mm, €5.5