Wow, thanks for all the good ideas. So here's what I did...
The last three inches of fuselage has a bulkhead installed which made
it the perfect place to isolate some weight.I cut the bottom sheeting
just behind the bulkhead and the removed that small piece of sheeting
back to the tail. I reinforced it with another small piece of balsa and
made a hatch out of it - easily secured with a few 4x1/2 screws.
I inserted enough lead to get it to balance then stuffed some foam to
fill the rest of the cavity, then installed the hatch. I left the lead
unglued in case I want to take some out after the first couple flights.
When I am happy with the performance I can glue the hatch back down.