Over on Hackaday, we've seen a post about Jay Doscher's clever 3D printed enclosure that can hold two RTL-SDR Blog V4 units and a USB hub. The enclosure is designed to make it easy to take the two units mobile, and the USB hub inside means that only one USB connection is required to run the two units.
Jay has also thought about cooling, allowing for space between the two dongles, and adding vent holes. He has also ensured that the SMA ports on the dongles are protected while allowing space to hand-tighten the connectors.
Jay writes that he has tested his enclosure with RTL-SDR Blog V4 units, but given that the dimensions of the V3 (and V2) are the same, it will work for those units too.