These little voltmeters are great

[Smurf Hunter]

https://www.amazon.com/dp/B0761MG9NS



Carl gave me one to use for a project (thank you Carl).  These little guys are inexpensive, simple to wire and look decent too.
When you buy the variety pack, I think you get up to 5 different colors.  I personally like the white or lighter colors as they stand out against the black in varying light conditions.



These are inexpensive enough that every battery box or similar enclosure could use them.  You mount from the backside of a surface, either with screws, silicon/glue, etc.
I plan to get more.

[Alan Georges]

Looks good, especially at $11 for a 5-pack.  *plonk* in the cart.  Thanks!

There are a several other good options too that popped up at the link: ones in housings ($7 for a 2-pack, still not bad), all red or green for the same $11, etc. 

[Carl]

  Another of my Ham secrets escapes to the masses. They are cool meters and I have run them for almost a year with no failures.

[Smurf Hunter]

  Another of my Ham secrets escapes to the masses. They are cool meters and I have run them for almost a year with no failures.

I plan to add one to my Astron 35amp.  I probably will just use velcro or a magnet.