Hi Damon
Here's a couple of ideas you might try. I used an 1100mil flat pack and with a bit of trimming on the back of the tank tray managed to get it to fit very nicely under the bottom rear of the tank. This is probably similar to what Tychoc did with the velcro tape. The only draw back is the tank must be removed to get the battery out. As per making a hatch the quickest way I could think of would be to cut FB-2 and add some 1/4" square braces and small screws to close the hatch. With this area being so tight I don't think you can get a square pack in there. Also I don't know if you could get the battery in the front half of the tank compartment as I already had FB-2 in place before I saw your post.
Are all of these kits coming out this tail heavy ??
Are you using the wheel pants ??
The reason I ask is that I will not be using them as I am using a larger 60 size gear, I checked the weight on the wheel pants and they came in at 3oz. Thought maybe not using the pants was contributing to the tail heaviness, Though they are not on the front of the plane they are in front of the CG balance point. I am not sure how far this would throw the balance to the rear.

Sorry I did not reply faster but I have been setting up this new computer and transferring everything from the old one [X(] I will get some more pics on tomorrow.
Dale