If you did want to fit a cover over it, all you'd have to do is make sure you don't cover over any air inlet vents or the cooling fan if it has one (not familiar with these machines).
But as Paul says, most extraction machines are designed with the fact in mind that they may be subject to getting water down the outsides of them, either from rain or cack-handed pouring from buckets.
I have a large vinyl tarpaulin in the van, I often just use that folded up into a quarter of its size, it kind of makes a roof for the machine!