I stole this off Zagnut on RC Groups. Not exactly like his but pretty close.
I will be putting ply triangles or balsa triangular stock between it and the inner fus sides for extra security.
It is intended for soft bungee launches (F3RES) of about 4kg pull.
All you need is a 3mm ply base, piano wire and a set of "chocolate block" connectors. Carbon tube of suitable sized inner diameter and a small piece of balsa or ply as a plinth for the carbon tube both coming from the scrap box. I have fibreglassed over these parts to tie it even further together. I used cyano throughout.
I have mounted the whole lot on a piece of crossgrain balsa identical to the bottom sheeting if the model. The ply base is as wide as the fus opening, the balsa as wide as the fus sides.
A couple of pictures that should explain what I mean.