Offset valve spring retainers or keepers ?

At some point, just getting the tool and arbor and turning the spring seat a bit deeper may be easier. You don't need to do a lot here. And maybe you can set the guide OD up for better seals at the same time.

And if you have a lot of time on your hands, you just make your own tooling... Customize a holesaw arbor use a couple different diameters of saws, and then your dremel to clean it up.