We have tried most all the above, except AGM batteries and slide out tray. Somewhere along the way, something happened to make me move on to the next thing to try. We have about 20 batteries to service so always looking for something easier or better.
Currently ( & have been for several years),
we cut a piece of flat belting about 5/8"x4", drill a hole near one end to store hanging on a nail.
Open only one battery at a time, stick it down to the plates, then look to see if it has about 3/4" water (or whatever you desire). Wipe off with a paper towel, then on to next cel.
Then to add water, we use a gallon jug with a pitcher handle (Probably a green tea jug). Get a 30" piece of clear plastic hose. Drill a slightly smaller hole in the lid of the tea jug and force the soft plastic hose into the hole. Fill the jug about 1/2 full (for ease of handling with one hand) of distilled water. Hold end of hose in each cell while tilting jug so water will run. Shortly, you will get good at guessing how long to let it run. Then check each cell again with your depth gauge.
When servicing a golf cart situation where you can stand up and look in, Start with a full jug.
Store jug with a bolt slipped into discharge end of hose to keep clean.
Also, when changed out batteries, and battery compartment needed attention. We cleaned it up, Repainted, then covered the floor with flat conveyer belting for the batteries to sit on. We did this a little over 5 years ago and have not had batteries out since, so can not tell you if we got the results we hope for or not. At least the top side looks dry and not corroded.