Did Minolta ever make an adapter for their manual focus lenses that would allow you to mount a filter on the lens bayonet when you reverse a lens for macro? Similar to the Nikon BR-3 for the F-Mount.
I filter in this circumstance by taping wratten squares over the inverted end. I never even knew Nikon made such a device.
I have always been happy with my chance landing on Minolta as my first slr, and the foundation of my slr's I use to this day.
Actually I use 3x3 rosco filters clipped from theer 3x5" designer swatch books. The most filters for lens and flash that $20-25 will ever give you.