Don't think there's a way of "filling" the slits.
Just need to hit the local lawn and garden shop and look for old ryobi's and swap the cylinder.

I'm gonna try to swap cylinders with my "currently in use for the lawn" ryobi with my newer twin ring that has slits....

I had bought a reconditioned one with single ring...and found it had the slits too....I was bummed...single ring with the slits...sigh...

4 Ryobis, now scatterred around the house in various stages of conversion....two twin rings, three slitted cylinders (one unknown)...

Maybe some phone calls are in order to local lawn repair shop...

CrazyHerb