SLFlt,

If the battery relocation isn't feasible and you have to add weight at the tail, you don't have to cut a hatch for this. Merely drill a 1/4" hole close to the rear on the underside. Generously coat small fishing weights with epoxy and drop into hole with the plane pointed up to allow the weight to drop rearward. I use the bullet shaped weights with a hole drilled through the center. (3/8 ounce works well)

You'll need to know how many weights will balance the plane before you start dropping them through the hole. Take into account the weight of the epoxy as well.

When the epoxy has cured, check the balance, and place a small patch over the 1/4" hole.