You might try cutting down the rubber gaiters that are on now- down to about 10 - 15 cm. long and see how that works. You can leave the bottom fastened tight to the forks and put a plastic zip tie around the top to hold it loosely against the fork tube. (That's really all the neoprene guards do anyway.)
If that doesn't work experiment with a section of inner tube in the same way. (the 'boot' doesn't need to be completely sealed, you can slice the tube open and leave the opening in the back of the fork tube.
.........shu
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