Bit of a minefield when it comes to Reducer/Flatteners for William Optics scopes I've found over the years. You may have already googled Megrez flatteners and possibly found a lot written about them on Cloudy Nights, Stargazers Lounge etc. and my overall impression is that is a hit and miss affair. I currently have a Megrez 110mm and bought the WO AFR-IV 0.8x R/F IV which doesn't come to focus (not enough inward travel) but I believe it is suitable for the Megrez 90. I've heard that the TeleVue TRF-2008 RF works so it may be worth researching some more. I also noticed in the Megrez 90 instruction manual that an RF is not shown, see
https://williamoptics.com/download/support/megrez90.pdf