Installing a battery maintainer on 12 v Prius C battery

I've been having problems with the 12 volt battery going to zero on me because I don't drive it on long enough trips to keep it charged up. The AAA battery guy is getting tired of me so he says I should install a battery maintainer on the 12 volt. It seems you need to do this at the battery under the seat and not at the special connection under the hood. But using clips is not really an option in the close space if you want to close it up again and leave the connection in place so you don't have to take the seat apart the next time. The battery maintainer I looked at has some screw on optional connectors but I can't figure out where you would attach them. The Toyota dealer service guy said they could install it but would charge me $150. Can anyone give me some advice about the right way to do this? Oh, also there are 0.8 amp and 1.0 amp maintainers. Any advice about which to buy?