What kind of adhesive or glue should I use to bind them?

I'm going to second the foam tape.   I use it all the time and it works well.

Ideally you wouldn't use glue, but double sided foam adhesive tape or similar. This reduces the chance of damaging the pouch, especially as the cell can swell throughout its life and needs some ability to move or flex to accommodate. Silicone is fine too, but takes a while to set and can be smelly. Probably not as big of an issue with smaller cells like this as they won't swell much. Hot glue is not ideal due to the thermal shock when applying it that could damage the material of the pouch and can loosen especially when it gets warm.

Second GL suggestion of double sided tap. Augh SH&^% always seem to happen and glue doesn't accommodate them,

I've used this stuff with success.

www.amazon.ca/.../ref=asc_df_B0833DT387

Superglue doesn't stick to all plastics. You'll need to be able to remove and replace the lipo when it gets puffy. 

Can get a hot glue gun at craft stores, hobby stores, hardware stores, dollar stores, etc...

Yes, double sided foam tape is another great suggestion. 

Okay, thanks for the experience.  I have put this in my cart:

https://hk.element14.com/pro-power/adsft12x10/tape-double-sided-foam-12mm-x/dp/3382579

3M also sells very sticky double-sided foam tapes; heat-resistant, can stick to polished tile, very sticky but very expensive, not for massive deployment.

Silicone sealant also sounds good.  It is anti-corrosion, elastic even after a long time of use.  Every window has it sealed.

The other advantage I forgot to mention is that when it comes time to replace the battery, foam tapes can be easily cut through with dental floss or piano wire, safely separating the battery from the case. After that, a little isopropyl alcohol and a scraper takes care of the residue.

Alternatively, 3M command adhesive strips that have a pull tab to detach are perhaps a good, albeit slightly expensive, option provided the space exists to use the pull tab.

- Gough

 Gough Lui 3M Command Strips!  Great recommendation!  The wife uses them for everything.  Why not me? 

robogary said:

but if you change batteries , you need to put velcro on the new one.

Not if you hot glue the centre of a double-sided Velcro strip to the case, leaving the two ends free to loop around the battery.

Could also use a nylon cable tie base with a cable tie or a Velcro cable tie.