Some time ago, I turned on the bath vent fan before a shower, only to have the fan blades turn into shrapnel. I have ignored the problem for a while since I have had many other things to occupy my time.
I have hated that noisy fan since new. It destroys TV when it is running. I decided to do something different.
I have a 5 1/2" square, 12v
muffin fan. I can easily sand off the non-functional corners of the muffin fan to fit it inside the approximately 6" vent stack. A little foam padding glued to the flat sides of the fan will easily stop any leakage around the fan. I'll "invent" mounting parts as needed.
I have a rally coming up in a few days. On return, I'll get out the ladder and swap fans.