Razor washers

bro thats the one i got the flimsy plastic one which is clear and bends as soon as any contact is made lool

was going to grab these 100 for £3.40

 
You can buy NEOPRENE RUBBER WASHERS from ebay for 5 times the price, but it can get obsessional.
Just buy the M5 10x1 black, and the experience will guide you. I've never broken one. I lost one down the toilet and didn't put my hand in for a penny. So don't believe the rumours about my being mean.