Its like a 2x4 only its 8" wide rather than 4" wide. You can use concrete/cement screws to screw into the top of the concrete blocks. You could also fill the top of a few blocks on each wall with cement, and sink an L bolt into the cement. Then you drill a hole in the board to go on top, and put a washer and nut on the end of the L bolt, holding it down tight to the concrete blocks.
Its not too decorative. It adds a wood finish and helps hide the edge of the liner.
Another option, you could get the 8x16 concrete pavers from home depot. They are designed to sit on top of concrete block walls, and the weight is probably enough that the liner wouldn't pull out from under them. Those you could just sit flat on top with nothing to secure them but their weight.