My cheek piece is a new product designed to fit the Ravin R9, R10, R15 and R20 and now the R26, R29 as well.  It's adjustable in height but limited to approximately a 3/16" range.  
It's specifically designed for folks who use a Burris Oracle X or
a Ravin jack plate below their scope.  But it will benefit anyone who uses a scope that mounts higher than normal above the crossbow scope rail.