Compared to a light weight park flyer most ducted fans fly fast & use a lot of power to do it so I asked myself a question. Is it possible to build a lightweight park flyer with a fan?
The emphasis would be a shrouded fan rather than ducting to emulate a turbo jet. So a bit of experimenting.
First the fan. 7 blade 76mm diam fixed directly to the bell of a 2812 1600kV out runner.

With a simple shroud on a test stand.

On a 3s LiPo it produces 6oz (170g) thrust drawing 13A which should fly a 40" span 12oz plane ok.
This was the sort of thing I had in mind.

Keeping weight to a minimum a test fan shroud made of a paper/depron/paper sandwich. It weighs 0.14oz (4g)

Of course this may not work at all!