In reply to Os_878:
Nalgene bottle, rolling pin, barbell, length of hard drainpipe, sturdy wine bottle (careful now), lacrosse ball, golf ball, lump hammer, pneumatic drill.
A barbell plate, kettlebell or flagstone to weight down any of the above during torture sessions can also help and, as Kelly Starret likes to point out, it means 'you can black out, and when you wake up you're still self releasing'. Great.