Salt doesnt evaporate. When you have surface agitation that ends up spraying water (even water that you cant really detect), it collects on an object, evaporates, and leaves the salt behind. This is what causes salt creep. You should be able to keep it to a minimum by keeping your surface agitation to a minimum. However, you will draw alot more oxygen by agitating the surface in a rolling fashon. You just have to find the right balance.
To answer the question. You should be fine to run the canopy the way you want. I would leave the top on in case of carpet surfing fishes. But you will cool it alot better with a fan pushing air out somewhere. It is kind of pointless to have alot of air being pushed at a high rate, and nothing blowing it out of the space at the same rate or higher, if that makes sense. The opened back of the canopy will help a little in this department though.