This no rear bumper thing is catching...

[Barrbeast]

Looking at the pics again, I'm wondering if I can use the tie down point as another connection...

 

[elty001]

ive got the same problem as where to connect my new bumper to the chassis(when its made)I want to bolt it just behind the rear wheel to give the corner some support.
there are 2 m8 captive nuts that hold the bit of tin to the edge of the standard plastic but not much for strength.
other option would be to clamp around the chassis with some thick plates.