Notice that I said VOLATIZE THE ENTIRE CONTENTS? You can't do that by knocking it over or leaving the cap off. It is still perfectly safe. I mean really guys, we did the math. Even directly volatilizing straight thinner or airbrush cleaner into a paint booth WILL NOT EXPLODE.
Now, I wouldn't POUR thinner onto the fan motor, but then it would be the electrical short that started the fire anyway. So you might want to make sure that your motor is above the level of the base of the booth. Which, by default, it ought to be because if your opening is the size of the motor, you're not going to move air properly.
So really, rather than speculate, go back in the archives and read the MULTIPLE discussions on this.
Here's a quick science lesson. I could. say, "hey, I've been using bathroom fans for nearly 20 years without a problem" but that is what we call "anecdotal evidence". As scientist who is VERY safety conscious, I can assure you that rather than do that, I and several other list members, investigated the flash points of all of the chemicals used, and then calculated their concentrations when volatilized. I airbrush quite safely using my homebuilt box with bathroom fans, that cost me a total of $50, built probably close to 20 years ago, not because it hasn't blown up yet, but because I KNOW that it will not. And all the while laughing at folks getting suckered into buying $200 fans...