I never thought of cast iron planter plates as a source for grates:Actually, I never saw planter plates that looked like these.
Those are cool!!!!!!!!! plus more!!!!! lol
If cast iron is a good option, I like floor drain covers. I've had to remove these by the dozen in parking ramps to clean the drains.They can be purchased in different diameters.
Here's an 8" grate
call that a genius finding...