My Burro (and the Escape) uses an Elixir mini-vent. It does not have a fan, but I have added a small one to it.
It is meant for a 9x9inch cutout. If this would fit your trailer, you might send me a PM, because I have modified it slightly and the changes may be useful for you.
I googled the Elixir mini-vent and that's pretty close to what I already have, but mine is made out of original aluminium. I also just saw your other thread
on the topic. But unless I missed something, the Elixir also seems to lack rain-shielding capability, n'est-ce pas? In that case, my preference would be to keep my existing vent, as I really appreciate the fact that it doesn't degrade over time and the spring-loaded cover still works like a charm after 34 years. That goes well with my quest to preserve as many original (read: durable, non-plastic) parts as possible. MaxxAir-type lids might do the trick if I can't find anything better, but seem disproportionally huge for such a small vent. As for the cutting option, I'm keeping it as a last resort.
I wouldn't mind finding out what you've done as far as adding a small fan (type, size, amps, noise level, etc.). Also, did you do anything to help with rain-shielding?