Just been in the shed attempting to make a cheap bottle filler. Started off with a standard rubber bung:

Marked up a rough shape:

Did a few cuts:

A quick spin on the drill with a file:

And Bobs your uncle:

BUT, there is a problem. When the bottle is held up against the bung, the pressure forces the beer out the valve higher up the tap. Dropping the pressure to almost nothing helped slightly, but I'm not convinced it'll work.
The end result:

Will leave a day in the bottle and open to check for carbonation levels.
Does anyone have a way of improving this? If this is doomed to failure, I'll just copy the "we don't need no stinking beer gun" guys on youtube....