Amazing images, thanks for sharing!
I've actually been doing a bit of research into this as I was hoping to try some of this oil/water photography next week. I've seen various different setups and your setup of a small glass on top of a phone is by far and away the most simple, will definitely give that a go!
what else do you think I could try to give an interesting effect
I'm not sure yet, I'll have a better idea when I try it. As a macro photographer I'm sure you are aware of Depth of Field, so would using a taller glass (pint glass etc) blur/soften the background more? Also when doing my 'research' I noticed lots of people recommending to use washing up liquid as well as oil... have you tried that?
other than that then just keep playing and experimenting... who knows what you'll see the next time you try this :-)
RE: Experiment - oil and water