Thank you to Walter P. for writing in and sharing with us his conversion of an old analog boombox into a fully functional wideband software defined radio based on an RTL-SDR dongle and upconverter.
Inside the boombox Walter stripped away the analog circuitry and replaced it with a new LCD screen, Raspberry Pi, RTL-SDR, upconverter and an audio amplifier. Four rotary switches on top of the radio are used to control the frequency, demod mode and volume, and there is also a numerical keypad which can be used to enter the frequency directly. 5V and HF antenna connectors have been added to the side, as well as an upconverter enable switch on top. Walter also added a Spyserver mode to the software, which allows you to connect to the radio over WiFi with SDR#, although he notes that using the integrated Pi WiFi module seems to introduce noise on the speakers.
If you're interested in building a similar device, Walter has provided the full Python code and installation instructions for his build.
Edit 09 May 19: It was pointed out that the word "ghettoblaster" could be considered offensive in some cultures. We have changed the word in our article to "boombox" and apologize for any unintended offence.