help with SUPER small scale Co2 diffuser

[951socal]

growing hairgrass in a spare container.

I fashioned this thing out of an old water treatment bottle, an air hose and an airstone.

this setup right not is only temporary. i just wanted to get some co2 in there NOW while i finish building the thing.

Im going to head to home depot when it opens here on the west coast go get a suction cup (one with a hole in it so i can zip-tie some sort of clear cup to it.

problem is.

WHAT CAN I USE TO SET IN THE WATER TO CAPTURE THE CO2?

any ideas
it has to be small and see through. not exactly clear, but it can't be opaque. and i have to be able to zip tie it to a suction cup.

pics of current setup: TEMPORARY

 

[maxwellag]

You could use the bottom (cut off) of a plastic bottle. Another thought- if you are growing dwarf hairgrass by itself, why not grow out of water? You could put it in a little container with some dirt and it would probably grow faster than underwater- and you wouldn't have to dose co2.