Cable ties will be fine. The reservoir is surrounded by so may other bits of car that it will never fall out even if not held in with anything.....
The most important thing is that it is now all back together & everything works including the headlamp wash wipe. When I first switched on after re-assembling everything I still had the low screenwash error message. Happily this turned out to be correct & when I filled up with screenwash the message disappeared.