The pilot reported that just prior to departing on the glider flight, an unsecured seat cushion was provided to his passenger in order to elevate his seating position and improve his view. After releasing from the tow plane, performing a 30-degree bank turn, and leveling the glider, the pilot was unable to move the control stick fully aft in the pitch axis. He attempted to troubleshoot the blocked flight control, and asked the passenger if he noted any obstructions, which he did not. During the subsequent forced landing, the pilot was unable to safely maneuver to the departure gliderport, and landed in an adjacent field where the glider struck a fence, which resulted in substantial damage to the fuselage. The pilot reported that his examination of the aft cockpit following the accident revealed the seat cushion had shifted during flight, blocked the control stick, and that the cushion's internal metal frame and the aft control stick displayed witness marks consistent with contact during flight.