Battery.

Don't know where you are but if in a not all too cold area, why not switch to a Lithium-Ion battery.
I have done this due to having the OEM alarm unit under the pillion seat and needing to also house the PowerTRONIC unit.
I have bought and mounted a W-Standard 12Ah Li-Ion battery creating enough space to pack the PT on top.

 

Don't know where you are but if in a not all too cold area, why not switch to a Lithium-Ion battery.
I have done this due to having the OEM alarm unit under the pillion seat and needing to also house the PowerTRONIC unit.
I have bought and mounted a W-Standard 12Ah Li-Ion battery creating enough space to pack the PT on top.

Hi so I’m looking for replacement batteries. Anyone know if the dimensions of the Yuasa YTX9-BS AGM Battery will fit our 2017 duke 390? The dimensions are 6.00 in. Length, 3.437 in. Width, 4.187 in. Height

I’m in New York so the weather gets pretty cold sometimes so I don’t really want to switch to a Lithium

 

I would highly recommend a Lithium ion one. I don't think the modern ones have any real issues starting in any weather you are likely to be actually riding in. They are massively lighter than the stock ones. Far more than you can imagine.

From shorai's website FAQ's

Q. Cold Weather Performance Expectations?

A. Down to about 20 degrees Fahrenheit (-7C) most users find that they can start normally on first crank. If your headlight comes on at key-ON, it is good for the batteries to flow some current before cranking in cold weather. The suggested headlight-on time before cranking depends on the temperature. If starting at 40F/5C, 30 seconds will help wake the battery and increase cranking performance. If at 0F/-17C, leave the lights on for 4~5 minutes before cranking. The result will be a better first crank, and longer battery life. If the engine fails to start on first crank, that first crank has warmed the battery, and the second attempt will be much stronger. Other accessories that can be turned on before cranking can also be used for this purpose, such as heated gear, radio, etc... Insuring that the battery is fully charged after storage also improves first-start performance in cold weather.

I'd look at landport batteries as these are far cheaper and just as good as the shorai ones. Note you will need to do something when fitting the lithiums as they are far smaller. Zip ties can work if you are clever.

I made a custom 3d printed bracket for mine, but I'm just odd like that lol

 

I looked into those, I guess I can use foam cut outs. But I have it down to shorai or yuasa. I’m leaning towards yuasa because it is cheaper and it has a pretty good reputation. I’m just not sure if the battery will fit because it is 6” in length compared to the stock battery that is 5.94”