Hello Stuart...many use regular plastic mesh gutter guard you can easily cut to size and found at most home improvement big box stores. I use very small zip ties to hold the end together and snip off excess.
The other thing you find as I did is I had to weigh down the bottom as some parents tend to be aggressive.
There is another option breeders use is galvanized plastic coated mesh which is easier to shape and heavier which is better at staying in place.
...Ralph