Allow me to formally welcome to the Association again 😀
I see this question exactly was asked on Cloudy nights forum in 2009. No-one seemed to have a good answer.
I’ve tried flatteners on telescopes they were not designed for, and they always improve the results over a non-flatted setup, though not necessarily as much as should be possible when using exactly the right thing for the job.
It might be best to find one or other of the lenses suggested in that thread second-hand and just try it out. It might not be fantastic but might be good enough to be getting on with without large expenditure.
Others may have other suggestions.