AGM's have very specific charging requirements. Unless the engine has a voltage regulator, not just a rectifier, it should not be used to charge the battery. Most modern engines will have a regulator.
My boat has an electric starter, but I found leaving my Garmin GPS/Map/Sona unit on would kill the battery over a 3 day weekend, before I rigged up a charging system. You could easily charge the battery every night though.
I have a larger battery box I am no longer using. Like this one. NOCO Group U1 Battery Box: BatteryMart.com
I could box it, weigh it, and you can cut me a shipping tag? Since there is zero profit, that is my offer.