If you can live with its price and size, the Sonos Move is an excellent portable wireless speaker that expands the play area of your Sonos system.
A solid portable speaker for an existing Sonos system; Sturdy, water-resistant, dustproof and voice-enabled (on a Wi-Fi network) with Google Assistant and Amazon Alexa; Up to 10 hours of playback; Autotuning system calibrates to any environment; The battery is replaceable.
Somewhat expensive, the Move is big and hefty for a portable speaker; Battery life can be adversely impacted by how you use it.