Maybe something like this below with a big USB power bank battery. It has an internal battery good for 10 hours, but it sounds like it can be charged (via a usb power bank battery ? ). Maybe go to a Best Buy to try and see if this is viable. Bring your audio source.
I've never used the one l linked below, I just searched for "collapsible Daisy chain speaker" but I have a similar one that I got for cheap from somewhere. Be warned that they tend to introduce noise/static which is noticeable when Alexa isn't outputting sound. So I'd recommend a Y audio splitter to send one feed to your stereo and one to your always on speaker.
AYL AC-48073 Portable Mini Capsule Speaker with Rechargeable Battery and Expandable Bass Resonator, Black https://www.amazon.com/dp/B007OYAVLI/ref=cm_sw_r_cp_apa_GNKcybA07ZXSB
i would think weight would be an issue but https://www.bose.com/en_us/products/speakers/wireless_speakers/soundlink_mini_ii.html#v=soundlink_mini_ii_carbon
However, i have one of these and love it https://www.amazon.com/gp/product/B007OYAVLI/ref=oh_aui_search_detailpage?ie=UTF8&psc=1
you can even string many of them together for better effect.