Remove the lower sheeting, Make a plywood hatch to fit where you removed the sheeting minus a bit to account for covering thickness. put a couple 1/4" square hardwood rails along the fuselage sides to bolt the hatch to. I would put some eyebolts or something on the hatch so you can use rubber bands to secure the battery. Some people might use velcro, but if the battery comes loose, you can kiss your plane goodbye.

Check those rubber bands at the beginning of every day at the field and if you live in a hot environment, I would check them every couple hours.