This is a macro diffuser I fashioned out of a gallon plastic milk jug. I used a caliper to measure the outside diameter of my lens, then used a compass to mark a circle that size on the plastic. I then used a razor knife to cut a hole just the right size.
The plastic diffuser slips over the front of the lens and is held in place by the tabs that normally hold the lens hood.
If you need to hold the diffuser more securely you could attach it to a filter step-up ring, but I find the slip-on approach works quite well.