A student in the second grade has spastic quadriplegia secondary to cerebral palsy and uses a power wheelchair for mobility. The OTR observes the student throwing and catching a ball with peers during gym class. As the student attempts to throw the ball, the trunk shifts laterally and the student has difficulty moving back to midline to continue the game. What modification to the wheelchair would be MOST EFFECTIVE for promoting postural stability to maintain a proper base of support while this student is throwing and catching the ball