If you have the option of running off a single cell, then also consider using a single AA cell instead of two AAA.  The energy store and weight of AA is about 2x of AAA.   :)

http://www.allaboutbatteries.com/Energy-tables.html

In Canada, you can get a bulk pack of batteries from a big box store at around the same price *regardless* of the actual cell size: i.e. D or AA or AAA   If this is the same for you, you reduce the battery cost and may be battery holders by factor of 2.

(9V being the exception as it is very expensive.  However if you use a factor in 6 cells inside, then it sort of works.  That's the reason why I hate 9V batteries.)

Thanks, that is an interesting idea.  I do like the idea of reducing the BoM count from 2 batteries to 1 battery.

Regarding battery holders I really like these clips https://m.distrelec.de/de/akku-batteriehalter-aaa-keystone-55/p/16961692

I guess in greater quantities they get quite cheap

Actually there are know defect issues with the keystone *style* metal holders made by MPD on my Mooshimeter.  The metal was sharp and it cuts into the plastic cover of my battery making contact with the outside case (i.e. negative terminal).  So there is a short at the positive terminal discharging the battery.

https://moosh.im/2016/09/short-circuiting-battery-problem/

I have used the following. The metals contacts are too stiff, so you do have to get the dimensions exactly right.  None were provided from the vendor however.

https://www.digikey.com/products/en?keywords=CB-AA-PC-NEG-ND

https://www.digikey.com/products/en?keywords=CB-AA-PC-POS-ND

I have used them in my project: 

https://hackaday.io/project/5903-sonar-for-the-visually-impaired

I think I have seen some SMT battery holders on Aliexpress, but they are not cheap.

@K.C. Lee ah, thats really bad but sounds like a quality control issue after all. Have used many of them but they might be the "real deal". At least they have no sharp edges.
Anyway, the holders you linked are waaaay more expensive! At least when using 2 batteries...

Then just add these: https://www.digikey.com/product-detail/en/mpd-memory-protection-devices/BC2AAAPC/BC2AAAPC-ND/2439242

Nice to know they have something a bit more reasonable now.  Back then the one I used was the cheapest at the time and their distinct shape makes it possible for the end user to feel the difference between the polarities which was important for the project.

Chinese battery case with cover + switch are around the $1 mark.  I actually used a 4AAA case took off 2 cells and put my electronics inside for a project a couple of years ago.  

Have you considered large button cell batteries instead? They are pretty safe (thick metal case and small capacity), not very heavy, already have around 3V and have a rechargeable option. They can usually be sourced much easier than any LiPo batteries, and are not that expensive. I was considering using LIR2450 batteries for my badge design, but then reverted to 3xAAA because size turned out to not be important in the end.

Thanks, I do like the idea of lightweight coincell batteries.  I wonder if there would be shipping restrictions since it is a Li-ion.

Personally I never had a problem with them — I suppose that they look so commonplace, that nobody gives them a second thought.

If you order them to drop ship to the destination, it becomes someone else's problem.  :)

Do however read up on the battery datasheet to make sure that the discharge current is within what the battery can handle.  These (non-rechargeable version) batteries have high internal resistance so not too good battery life if you want a few tens of mA.