Hi,
I think you will find most d/f fliers experience is of either the 5" Ramtec here in europe
or the Dynamax in the US, the Ramtec suits the OS91 that most people use the best, but the Dynamax appears to allow a slightly smaller inlet to be practical. If you are looking for a smaller fan unit then your best bet would be the Thorpe Brothers here in the UK, I may be wrong but I dont think anyone else produces a suitable unit anymore. Their website www.thorjet.co.uk appears to be down attm but their tel no is +44 1572 822291, they have never been less than helpful so you could give them a call??? Mike