It's a tight fit for a group-24 for sure, but it should fit, ours did. If I remember, I think I removed the strap. There seems that there was a wood support that ran across the access, besides, there isn't any way for the battery to fall out short of turning the boat up-side-down, and if that happens, you've got other problems to worry about. Before we sold the boat, I was playing with the idea of cutting out the factory battery box so I could install a group-27. But in reality, we never had any power issues with a group-24 running an auto-pilot and VHF all day long.
Here's an idea.....for a mooring light, have you thought of those solar powered LED garden lights? They charge all day, and last all night. We have one that fits into a fishing rod holder that's fastened to the stern pulpit.
Great boat by the way!
Don