I'm looking for some tubing of a slightly smaller diameter than "40mm"
waste pipe, to fit tightly into the neck of a bottle (to construct an
aquarium filter)
PVC/PP/... would be OK.

However, the '40mm' pipe I measure is all a little bit more, around 40.3,
to 40.5, which is a bit much.
39.5 would fit very tightly, 40.5 is too much.

Can anyone point me at a source?
I can heat and stretch it to reduce diameter, but it's hard to do
consistently.