I just bought a dozen Grolsch (beer brand) rubber gaskets from ebay for $3.50. They are thick rubber that slides over your strap button with the strap on. Easy to put on, easy to take off and so far they seem to work just fine. They are orange though. I used to use mechanical strap locks but didn't really care for them.
Why buy the gaskets on eBay buy yourself a 6 pack and harvest them yourself haha