It's not the first time it's happened, either.
So I thought it was about time to do something about it.
I started with one of those plastic 25 litre drums, the sort you can (well, I can) easily get from a skip.
Then I made a cardboard template, checking the fit on the frame.
Then I cut out two pieces, cleverly using the curve of the drum to make an inwardly-curving forward edge on the guards.
And then I installed them, using tie-wraps.