Start with an assembly consisting of beam pipe cover barrel ladder support cone endcap radial outer cover cylinder endcap electronics carrier board You can now mount the barrel ladders. Ladders are fixed to support cone B, cables are guided along B and C, and electronics are mounted to the inside of the carrier board D.
An outer cover is mounted over the assembly, which can have either 2 or 4 layers in it. The outer surface of the carrier board is still visible.
An endplate cove F is mounted. Barrel is ready to run.
Alternatively, you can turn the assembly over, and mount the endcaps inside cone B and cylinder C, with electronics mounted on the other side of carrier board D. After this, mount the endplate F (from the previous picture). In addition, you can add beampipe covers over the endcaps. These go between beampipe cover A (from the top picture) to the endplates F.

The carrier boards D are the main anchor plates. These can be made very stiff and hold the connections to the external mount points.

For strength, a series of posts going radially out from the joint between B and C can connect to outer cover C. The posts need to go in between barrel cables, pipes etc. Also posts parallel to the beampipe connecting outer cover E to the carrier boards, at the inner radius (~20cm) of E.

The halves have little strength against twisting around the z-axis, but are very strong against twist after the two halves are locked together.