I also thought I would make and install a ballast tube in Sol's fuse. It just happened that I had half a carbon tiller extension left over from my Cat racing day's so out with the saw and down to business.
One end is sealed with a wooden dowel and a groove was cut out
The groove is housed over the GRP strengthened wing joiner plate. The groove is deeper than required to allow a little for and aft movement without damaging the joiner plate (that's the theory)
The loading end. I added another GRP bulkhead behind the former to support the tube, I haven't made up my mind yet on the most practical way to make good the end to allow easy loading and securely hold the ballast in place. Watch this space!