REI Flash daypack, not so good water bottle pockets, can fix?

Looks like, for the pocket to work properly, the bag needs to be pretty full especially with something as heavy as a water bottle. I’d bet an elastic cord loop, clipped or sewn to the mesh could help pull the bottle down into to pocket.

Here an example of a clip or a safety pin could work in a pinch. A small sewn in loop might give an attachment point for other things that need to be placed hung outside the bag.

With “not so good water bottle pockets”, maybe you need some not-so-good water bottles?  i.e. smaller, shorter water bottles.  Maybe that’s incompatible with your water treatment scheme that needs a particular screw top or a largish opening for a SteriPen, but otherwise, try a little dumpster diving at the recycling center and try a number of different skills / sizes / heights / widths.

I’m not totally sure of the scale, but the first thing that came to my mind was a somewhat short, squat bottle and that square-cross-section bottles are often more secure in a side pocket.  Something like the smaller, shorter Fiji brand bottles:

As for modifying your pack: try a safety pin first to snug up the top of the pocket and see if that gives you enough tension to hold a bottle reliably.  If so, it would just be a few stitches to take up some slack.  Or to add some elastic (stretched when you sew it on) to the top of the pocket.

Or return it and test other packs in the store with large, full, round water bottles.  I certainly like the option of having large water bottles convenient but secure..

I’ve had acceptable (to me) results with that packs and .75 liter Smartwater bottles. I’m sure shorter bottles would be even better. I carry a larger 1 liter bottle inside the pack and transfer water to the smaller bottle occasionally during breaks.

It’s not a perfect pack but I got it on clearance for ~$30 and I think it really nails the price/performance ratio at that cost.

YMMV