By inserting rebar, the form can be filled as little as 6 in. and added to later, a good use for leftover concrete from other jobs. "The only thing anyone is going to see is the front of the block, so if the sides are a little marred or have pour lines, it doesn't matter," notes Stocke.

Jean Feingold is a Gainesville, Fla.-based freelance writer who frequently covers concrete and other construction-related topics.