Harry Bloomfield wrote:
[snip]

Cut the top of the batten at an angle, such that the lip of the bath
is drawn against the wall as it is lowered. To fix it, cut vertical
slots for the fixings and use screws plus washers. Leave the screws
sufficiently loose that the weight of the bath can push it down, then
temporarily fit the bath in place. Remove without disturbing the
batten, then fully tighten the screws and perhaps add more fixings.

Ain't got a clue what your getting at?
What fixings?