You are not only going to have to build a hatch, but you are going to have to build a platform that is secure enough to deal with the weight of the battery. Although it's not that heavy under normal conditions, G-forces will "increase" its weight so you might want to construct something out of 1/4 x 1/4 stock and covered with 1/8 ply, shape it like the battery, attach the battery with velcro, perhaps a velcro strap, and secure that where you want it to be.

I'd make it long enough to be able to slide the battery frontward and rearward to accomodate minor adjustments in CG location (no fuel on board for this, by the way).

So, work this out so that you do this during the kit building process.

Now as far as a hatch is concerned, well, some sort of lightly reinforced panel that you make fashioned similar to the plaform, but with tabs that slide in. Make up the hatch hole in the fuselage with a little flush mounted lip around the edges that the hatch rests on, then secure it on one end with the tab that slides into the fuselage, and perhaps a servo screw or two at the other end to hold it in place. Make this hatch similar to how you make the platform, but perhaps out of some light ply. You don't really want to make it entirely out of balsa, but there is no reason that the cover itself can't be balsa as long as the supporting portion, the frame, in other words, is made out of something that will give it structure and some light strength.

That's how I'd do it if I was making it. It's glow, so a what little weight you add should not matter all that much for the added ounce or so.

That's my take. I'm sure there will be others.

CGr.