Ideas for DIY auto top off?

Hi guys, thanks for all the solutions. I will see which one suits my situation the best. I won't be able to use any siphon methods or methods that take up a lot of space because my new apartment is very small. That is the reason why I'm not running a sump in the new setup so that I can leave the cabinet space open for buckets and supplies etc. I planned to have like a 2.5 gallon tank in the stand and use that as the water source. It's a 16 gallon all-in-one cube so if I used the siphon method the water source would have to be very high up since the RO water needs to drain into the rear chambers. I need a compact solution that's under $80 or else the JBJ ATO would be the way to go I guess.