This sized bottle seems to be best. Also the shape of being a bit wider rather than say the typical 300ml water ones (they also tend to buckle with the weight of a rat)
I suppose you could drill holes, but bottle plastic, even the thicker stuff around the rim, is really quite thin.
I used a metal skewer heated up on the hob - took a couple of minute to do them all
2 pairs of holes [opposite each other] at the base end gives quite a sturdy perch. This sort of width means they're near enough that you don't need excessive amounts of tie, but there is some curve in the bottle to make threading through easy enough.
A hole on each side at the top makes for a more wobbly perch - so increased difficulty for the more agile rat, or meaner owner
The thicker plastic around the rim means that even with the smaller diameter there's enough stregth for the bottle not to buckle with the weight of a rat or two.
