I have been going back and forth between an inexpensive 400 grit diamond sharpening plate, which is flat but thin, so I cannot apply much pressure for fear it will bend, and a flat piece of wood I ran through my jointer with sandpaper, which works, but the sandpaper loads quickly. I will likely buy a Trend 300/1000 grit plate, which is what Rob Cosman uses (the 300 grit side)
Trend Diamond Sharpening Stone Kit, 8 x 3 Inch Double Sided (300/1000 Grit) Bench Stone with Pouch, Cleaning Block & Non-Slip Mat, DWS/CP8/FC https://a.co/d/4E8lSsX