I have an open cab, non-ROPS tractor.
I'm considering buying a ROPS and then getting a sheet of diamond-mesh sheet metal, cutting it to the shape of the ROPS and then using square U-bolts to 4 point hold it there.
This would give me the flexibility of being able to see behind me and the confidence that I have some protection between me and whatever decides to fling in my direction.
If there are no after-market or kubota provided options, you might want to consider it. It will probably cost you a decent amount, but from what I have
read, it's not uncommon for brush cutters to throw stuff at the tractor.