At the cost of some storage space, you could use part of the side locker. If you want to close the shutter fully, then the pump goes a bit of a way in to the locker, but by having it that bit further out you can have the pump accessible all the time.
In the case of the 3781 ladder truck, if you remove the side slide out storage drawer under the chassis you can fit a pump in, either the PM pump or one of your own design, although you lose storage again.
Just some ideas.