Hello,
You might take a look @ Sill Masters - Premium Quality Vinyl Window Sills for a solid vinyl window sill. You can glue this right to the block with poly urethane construction adhesive. The sills are waterproof.
Hello,
You might take a look @ Sill Masters - Premium Quality Vinyl Window Sills for solid vinyl window sills. You can also cut these and use them for jamb extensions and/or casing around any moisture areas like basements, shower windows or kitchen windows