Raspberry Pi mock radio test using a pair of DAC/ADC chips, a few Raspberry Pi 3's, and absolutely no duct tape.
If you don't want to compile on a Raspberry Pi and you don't already have a cross compiler setup yet, the script arm-cross-install.sh will set up everything you need. It needs a little bit of space to work with -- 250 MB download, 6 GB total space, but the final install is 900 MB.