You can use the water bottle, but be very careful of the edges, if they're sharp your betta can get stuck in there and be seriously injured. You'd be shocked what they try to cram their little bodies into...lol!
You can also use a piece of sponge/foam, just stick it into the outflow on your filter and let the bottom drape into the tank. This will cut the flow down to almost nothing and gives more BB for the cycle as well, which is nice. :) And they can't get hurt on it, which is another plus. :) You can glue the sponge to the filter, you can rubberband it on, or on some filters you can just shove it in there and it'll stay put.
Either option works, but I wanted to give you the B option just so that you could have some more information. :)