Bathroom Extractor Fan

Hello all,
I have not posted before but have a look regularly for tips etc. Here is my problem, I hope someone can advise me.We have moved into quite an old house that has a lot
of condensation on the bathroom ceiling.
Part of the ceiling is sloped. In this part of the
ceiling is a hole, an air vent, which is a solid pipe
which goes up vertically through the attic space and
then out through the roof of the house.
Although there is good air flow through this pipe, we
feel it may not be adequate, especially in the winter.
The problem we have is that an ordinary shop bought
extractor fan will not fit properly into the hole
because the ceiling is at an angle.
All the fans we have seen have the fan and the cover
as one solid component, so that when they are
inserted, the cover protrudes and won't lie flat on
the ceiling.
What we ideally need are a fan that can be inserted
and a cover that can be fixed to the ceiling
seperately.
Also, with this set up, would there be a problem of
the condensation dripping back down into the fan?
I have a diagram of the pipe but I dont know how to attach.I would appreciate any advice on where to buy a fan that will fit etc.
Thanks

Why not build a box around the bottom of the existing air vent, this would give you a flat surface on which to mount the fan.