An early diffuser that was quite popular was to use a cellphone anti-glare screen protector. They are easily cut with scissors and you get several from each package. They peel off if you don't like them.
Another that was popular was to cut a disk from a plastic milk jug.
The techniques you use depend on the tools you have at hand. Since I have a lathe, I'd make a Delrin (plastic) collar that press fits on the bezel, and mount a frosted plastic lens in it.
Daniel
We want pictures when you are done.
Dan