I know that it's not really what the OP asked, but the topic reminded me of something so I thought I'd share it in case it's of interest.
The attached circuit is a constant-off-time controller designed with generic parts. In this case it's configured as a simple boost converter, but the output coil could of course easily become a flyback transformer. I built it and it worked (I've got no record of how it performed though) although I seem to remember having some trouble with EMI...
This isn't a true "constant" off-time design, as the off time is decreased as the output voltage rises, in order to keep the output ripple roughly constant. Off-time could easily be fixed by returning R1 to the input rather than the output.
To be honest I'd forgotten all about it, but it seems that I also did some maths on the circuit - I've uploaded that too, but (as the contributors here know) my maths is poor so don't rely on it!