A workmate with a nissan snorkel on a GU7 used a sock type (actually took 2) on a recent trip, clogged it, swapped it, cleaned it. Obviously stopped lots of crap getting to the filter.
If you can turn the top to face backwards (very noisy) when on tracks you will stop a lot of debris, but if you can turn it around you probably can fit a centrifugal pre filter, I have one that I used for the GQ (steel snorkel) that I swapped over as we left the black top.
Graham