Yeah, a rigid bottle would be better, the weight of the water is just deforming the plastic it sounds like. Maybe a shaped bottle like a 2L coke bottle would work better due to being more rigid. They still kinda oval a bit though.
A green glass wine bottle would probably look really cool if you backlit it.
I used the same sort of thing. But the screw in cap at the top leaked and let in air, so I replaced it with a much more professionalish black rubber plug thingy. It looks better than a 2 liter bottle, but that damn valve alone cost me $7.50 ! ! !