I noticed that the opening is smaller than the compartment. I will just un-screw the door and put the AGM's in there. Then I will put the door back and re-caulk it. Since they will be AGM's I won't have to service them so no need to take them out. This is all assuming that I am living in a perfect world!
That D4 looks like a beast but would probably serve me well. However, I do like the idea of having more than one battery in case something goes sour.
How much did the D4 cost you? Any equivalents that are not Lifeline Brand(Trojan? They are not cheap but I do understand you get what you pay for.