I was notorious for letting my feet go too far through stirrups. I now how Cheesegrater ones from Shires (I think) which work a treat. Before I got those I tried some of the cheese grater 'tread' bit that just slotted onto your existing stirrups. Trouble was, they didn't fit very snuggly and rattled. At that point Mud monster was very spooky and didn't like the rattle.