My suggestion is not to cheap out on an ATO. The last thing you want is for the ATO to fail in the open position and end up having all your water entering the tank in one shot. It happens quite a bit. Saying that, using a 5G container like you are is an excellent decision because 5G is not going to do excessive damage to your tank.
I like the Tunze ATO. Is uses a sensor to dictate water level and if it fails for whatever reason, there is a float switch as a back-up. I use the Elos Osmolator. It works great. The unit uses a heat sensor to control the water level. This means no matter how dirty it gets, it will work. Another nice feature is it slowly adds the ATO water to your tank by running every 15 minutes to top up. I love it as it keeps my water level bang on all the time, which my skimmer needs to function properly. It also uses a PLC that shuts down the pump if the water flow is restricted (kinked hose) or if the resevoir is empty to avoid damage to the pump. On the down side, there is no fail safe like the Tunze unit in case it fails open.
Both units run at around $200 and come with the pump and holders. Not cheap, but, you get your monies worth with more piece of mind. One unit to avoid is the JBJ ATO!!! Had one, failed open, and flooded my tank. Luckily, I was just randomly looking in my sump and noticed my sump nearly overflowing. No casualties but it could have been bad, real bad!!! That's why I went down to a 5G resevoir from a 25G resevoir. They are built cheap and cannot handle the corrosive salt.
