Do you have the snow dam installed in this pic? A good filter would have to go around the top of that because it's sealed with foam along the bottom.
Funky ideas people .
Let's see what we are dealing with here...
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The photo is taken from the right side with the wheel off. You can see the new fan and the new rollbar going around it. The surface below is the floor pan and above is the floor of the boot (aka trunk).
Given the limited space and the new rollbar that curves around the fan (and rotates up and down a few cms as the suspension flexes) some creativity will be required to do much more than a cloth covering.
As the new fan draws air in from either side and also from the centre if the idea is to channel air to the fan from a single standard car air filter the fan covering will need to box the fan without fowling the rollbar, and then collect air from below or above in some kind of duct. There is enough space for a good sized airfilter behind the fan and rollbar.
A proper air filter does sound best. For now the cloth cover is a good first-pass as I can get it done quickly and test if it throttles the fan too much.