Are you trying to make it so that your DIY filter has no bypass? That is, so that the outlet seals against the neck of the water bottle? I don't know what kind of setup you've got to move water to and from the tank, but you'd want to be really really sure you don't wind up with a clogged up bottle and the overflow getting stopped up completely. Just trying to avoid a 2g flood. :-)
I like sharkfood's idea better though honestly, just turn it into a 2g filter.