Nice work Paul! Love to see the equivalent video with aluminium uprights!
Are the holes less likely to shear with thicker spacers?
Wonder if the white piece or some other piece could be designed so that it moved rather than failed, without it moving under normal riding conditions? Picture something like the white piece that was a hook, so an impact would cause the bolt to force out of the hook allowing the tower to move hopefully without destroying the white piece. You could then either unbolt or force it back into the hook and keep going? Or something like a string of holes joined together but where the holes touch its say half the thickness of the bolt so the plastic has to compress to allow the bolt past it.
Anyhow, food for thought.
I was going to run with 10mm thick HDPE but based on your testing it seems ~6mm/ 1/4" will hold up just fine!
Keep it coming!!