Reddit mentions of YDL 3.7V 3000mAh 704070 Lipo Battery Rechargeable Lithium Polymer ion Battery Pack with JST Connector

This is the one I currently use. Unfortunately, it’s not currently in stock from the seller I purchased it from. But, I’m sure it’s just a generic speaker the company just slapped their logo on. So, you can probably another similar one. The only fault I found with this one is the placement of the suction cups. Unlike smaller, similar-looking speakers, it has 3 smaller suction cups, instead of one bigger one (like the next one I’ll mention). And the suction cups aren’t lined up with the face of the speaker. So, if you have
subway tiles in your shower, like I do, you either have to trust that 2/3 suction cups will support it, or mount it at an angle (the LCD screen won’t be level). Personally, I just made a bracket out of plexiglass that repositioned the suction cups so that all 3 were secure and the LCD screen was level.

This is the first shower speaker I had, which also has Siri support (double tap the phone button). But, my issue with this one is that there’s really no battery indicator. There is an audible battery indicator, when the battery is low. But after a few cycles of the battery, and it’s degraded a bit, that indicator only sounds about a minute before it dies. So, I’d just step into the shower and it would indicate the battery was low, then the battery would die and it would shut off a minute later... Luckily, replacing the battery in both models is really easy if you know how to use a screwdriver. They both use a common JST battery connector, so there’s no soldering involved. But, when the battery really died on the first one, I decided to just replace it with one that had a visual battery indicator on it. And, even with that one, the battery died after about 2 years of every day use. So I replaced it with
this battery, which is about 3x the size of the original battery (it just barely fit) and now a single charge will last me a month of roughly an hour of use every day.

I've been experimenting with this battery: https://www.amazon.com/dp/B07BTV1SCB

It's <2mm taller than the original battery. This battery is ~6mm less wide. With the battery protection board, it's about 4mm too tall (covers where the battery's ribbon cable is). I've been playing around with putting the protection board on its side, and adding wires to the protection board. It's still a super tight fit along portrait axis of the phone...but it still closes with the battery being just a little taller, and all the backplate contacts still make a connection.

To be super honest...a Nexus 6 is only $120ish nowadays...so I'm really thinking about abandoning this project altogether and getting a Nexus 6.