This has jogged my memory that mine has gone AWOL.
Its about a half inch, as I recall, until someone chimes in with a more accurate thickness.
One side is slightly curved, which moves it away from a simple manufacture.
Although may not be strictly necessary.?
Most folks just seem to make something up as required, no-one carries them ?
You can use washers to make up a suitable stack to test with, so the guard isn't stressed when its all bolted down.