you are boosing from 24V to 400V?
You dont need isolation?
If so, then cascaded boost is good for you.
I now go to find it in my files.
As below attached
It should be possible to do a real cheap flyback solution, driving it from a NE555 chip with some means of maximum voltage shut-off. The OP has not provided more details, but I don't see that it requires very tight output voltage regulation.
The EFD cores are very cheap and can be ordered pre-gapped. The IRF7495 is about $1 US. One will still have to be careful with winding the transformer using sandwiched layers for good coupling to keep the leakage inductance under control.
Here is a possible way this idea may be implemented. The 555 frequency should be set very close to 100 kHz and 50% duty cycle. The frequency can be adjusted by trimming R12.
The 100 n capacitor across the output is just to speed up simulation time. In practice, this will be a larger value.