Having put the main section together I designed and added some accessories to the assembly, I wanted to have a shelf on one end to hang tools like pliers off of so again I designed and cut this using FreeCAD (see the main details for a video on using the LC interlocking workbench to automatically add tab geometries to your projects). 

I then continued to CNC rout this part and asseble and glue it onto the main body, it's certainly strong enough as it is but I would probably incorporate this into the main design next time adding tabs into the actual body of the TTTTT. I considered adding one of these to each end of the TTTTT but I like that you can push the flat back of the TTTTT up against a wall or back edge of a desk which you wouldn't be able to do if there were two of these.