http://www.batteryholders.com/suba_holders.shtm
These are just holders but it's a start
Thats a project box with a battery holder inside of it.
Here is a series holder for 7.4v:
Battery holder: Li-Ion 18650 Battery Holder (2S1P) With 2.6" long 20AWG & PCB
Parallel for 3.7v:
Battery holder: Li-Ion 18650 Battery Holder (1S2P) With 2.6" long 20AWG & PCB
Not sure how you're wanting to get to 5v but one of these will work. They have PCB on them so check the specs to see if they'll do what you need. I guess you could remove the PCB if you are using protected batts.
Here is a parallel box not sure how hard it would be to make it a series box. 18650 battery case 2 x parallel connection DIY case | eBay
Where can I find a battery box for 2 x 18650 batteries?
I'd like to build a 5V BoxMod with 2 x 18650 batts.
I can't find it anywhere. Even MadVapes doesn't sell this kind of box.
What kind of box is it? Lots of different sizes but doesn't say what kind of material. Thanks!
Robert