Can a portable bedroom fan be slowed down?
I have once made a lamp with a light dimmer enclosed in a PVC box. If you get a box and make holes for wires then wire a fan dimmer to it, you can precisely control it's speed. But make sure it is dimmable. If it doesnt say "non-dimmable" or something like that, it should be dimmable.
EDIT: Since it has an integrated dimmer, it is very likely to be dimmable.