I've decided on a good method ( I think) for fixing the batteries into the nose of a plank (or any other fuse for that matter), which the nose is long with no canopy or nose cone.
I first determined the position to obtain the required CG position by taping them externally. Then I cut a strip of coreflute to the length that would support them and extend aft far enough to fix that. I fixed the batteries to the coreflute strip with hot glue and cable ties. Then slid them in, checked the CG and when right, I hot glued the after end of the strip where it was accessible.
The only thing remaining was to push up a cube of foam rubber between the batteries and the top of the fuse to prevent them bouncing up and down.

They can be removed easily if and when required.

Jim.