Sorry about the delay guys, I'm still tweaking the design a bit.
This is as it is right now compared to the 5gal tank. The one to the right would be about 3.5 gallons. I think I could make a few more changes to get it a bit shorter if needed. I'm limited on top space; trying to keep the cap, battery, and fittings in optimal locations. What do you guys think? Smaller still?
This is the cap I want to go with:
