The JBJ isn't much more than a packaged DIY topoff kit anyway.
Easiest in your case as it sounds like you are having serious evaporation issues is to look into the cause of evaporation before spending any money. You could just have to much water splashing around where it shouldn't be. Or possibly a small leak you aren't even aware of. After figuring out if this is the case, you also should make sure you aren't getting salt creep which will further affect your salinity issues.
There isn't much to a DIY system to be afraid of. The simplest can be made from 3 parts and only takes a little math to work properly. The parts are a quality 3-prong digital timer, aqua lifter aw-20, and some aquarium tubing. You just need to figure how much water you evaporate in a day, time how long it takes the aqua lifter to pump that much water, and split that time up over how many times you want the tank to be topped off thru out the day. No messing with glitchy float switches, and about 1/4 the cost of the JBJ kit.