Keeping grit out of your cooling fans

[Jeb]

I run all my rc's on dirt and gravel. The cooling fans on the esc's and motors get binded with grit from our terrain. When it's wet I run tight fitted buggy covers under the truck bodies to keep the grit and elements out of electronics and 4x4 gearing, but when it's dry dust and grit still stall the cooling fans. I've tried putting metal screen under the fan covers which helped a lot, but they still occasionally bind and I don't always have access to compressed air at the track. What do you use to keep fans from binding? I was thinking about trying a peice of air exchange filter used in heating and air conditioning systems. Plastic would melt and the air still needs to move out cleanly, but not let grit in.