I bought a cheap Onewheel plus for testing the different range extenders.
Since the Plus is out of warranty long ago, it's a good platform to hack as my XR is still under warranty and not much need to extend the range anyways.

-The extenders are home built based on commercial types.
-Parallel extender where the Onewheel battery is accessed .
-Xlr type where charging through the charging port while riding. Seen by the Plus as regeneration.

Thoughts and experience:
--- Parallel extender using a 16 -18650 cells is tricky as both batteries should be the same state of charge and not at 100%.
My mistake was the batteries were at 100% , the board went into "overcharge" warning and went into non balance mode.
The board still wouldn't balance after disconnecting the auxiliary battery, but after leaving the board with the lights on for 10 minutes, it recovered and worked fine.
---Xlr system worked great as it will add to the battery percentage slowly. Uphills will pull the percentage down, but evens out.

My own analysis:
-Parallel system , less parts, more work to install.
-Xlr system, more parts and cost ( MTP-7210 solar controller, Hoverboard battery and charger). Very easy to install.

Note the Xlr system won't charge the board if it isn't turn on.
To charge the board without it being turned on, a different Xlr connector is used where the center pin is powered up.

My gawd, now that you pointed it out and I'm seeing the package in a different light, it does look like something menacing.
The grey bag looks questionable and the cigar lighter plug looks like a push button.
Perhaps a defibrillator ? HaHa

I put together this package for charging a Pint. Since the pint has limited range and if you're going somewhere with no electrical outlet, perhaps this can help.
This concept is available commercially and is not new. The new versions have a higher charge rate, I believe ,as they use a higher amperage dc- dc converter.
This version uses a Mtp 7210a dc-dc converter and the observed charging amperage never approached 3 amps.
-It took 45 min to bring the Pint to full charge from 60%
-It is fairly compact
-Supposed to work from as low as 12 volts from a power supply(cigar lighter or power outlet in a car) I will try it out and report back. If it works, then don't have to carry around a inverter and Onewheel charger.
-Uses common 36 volt Hoverboard battery.
-Beats walking if you run out of juice in a remote area.
It's highly unlikely I would ever use the Hoverboard battery option as I live and ride in the city, however the car charger would be great.

@Glyph
Ah. Misread your reply.

Excellent choice! Good idea attaching it to your wrist guard, I get 50/50 putting on my wrist guards. Works good for you riding regular?

My biggest fear whether riding Onewheel on the street is getting hit from behind or vehicles getting too close.
I ride goofy so it's not too bad at least in North America where we drive on the right hand side.
I've tried a helmet mirror, but found it hard to focus(too small) and hold my head steady until I can get a clear picture.
I used a larger strap on mirror meant for a bicycle and put it on my left hand ring finger, I can get a great view.
I used a brand name mirror, but found the convex was too severe , I sourced a cheap offshore knockoff with less convex.
The horn is a bicycle horn is good for alerting pedestrians, but no good for warding off cars. The horn has 3 tones, the circus clown tone always get a smile from pedestrians.
Sorry, don't know if the mirror will work for regular foot riders.

This is knockoff mirror.

This is a brand name mirror.

I made a set of Pint float plates because the present ones for sale do not have a front carrying handle like the XR.
Also commercial ones don't have a kickup for stopping and front hits.
A heat gun, a vise and a wooden form to mold the handle. Next set I make will have a larger handle.
Probably the design of the Pint didn't allow for a recessed handle. It's a small package.

For those that have a Pint, you can cut the charging time in half by buying the Ultracharger . For those that have an XR and a Pint, there is a third party adapter. The third party adapter uses the standard XR charger NOT the XR Hypercharger.
Being the one to experiment, I decided to purchase a few components and make my own adapter.
It does work , but I had to check polarity many times before plugging in. I've let the "smoke" out of many electronic devices.
I do not recommend anyone else to fallow my lead and I will not give instructions.
I do recommend the Ultracharger for the Pint .

@LidPhones
I don't seem to have any problems with connectivity .
I'm on a iphone , no problems so far.
I can't get over how quick the turns are with the Pint, sneeze and you've done a 360.