You can easily set something up in the closet you have next to the tank. Here is what I would do.
1. Figure out how large of a container you are willing to put in the closet. One thing to consider is to plan for something that will last you 3-4 days if your R/O unit is shut off. You don't want to put in a 50 gal water storage container due to the possibility that your ATO could stick open and do two things. Flood your sump is the first thing and the second is over dilute the salinity in your water. I am pretty sure you don't want either of those things to happen. They can happen, I assure you of that.
2. Decide on which ATO device you want to use. There are a lot out there. Personally I use the Reef Fanatic unit and the unit itself has been great.
3. You will need a float valve for your ATO water storage container that will connect your RO unit. This will control the amount of water that will be stored in the container.
4. You will need a small powerhead to pump your top off water from the storage tank to your sump. I have used small mag 2 pumps, maxi jet pumps and other similar power heads in the past. This pump will stay in the top off water storage container at all times. Once you decide on the pump you will use then you can figure out what size hose you will need to run to the sump. This powerhead will plug into an outlet that comes on the ATO unit. It will be totally controlled by the ATO unit.
5. By the appearance of your sump you will place the float switch that comes with wichever ATO device you chose in the same chamber as your return pump that goes up to your 90 gal tank. Most of the float switches on that come with these units come with suction cups. I suggest you find some sort of magnet holder to hold your float switch in its desired location. I have used old algea scrapper magnets and just zip tie the float switch to it. This gives you great flexability for adjusting your water level.
