I wouldnt(and didnt) do it like that guy in the thread.
First, the bondo is going to absorb water over time, causing the metal to rust, and sooner or later it will swell and pop out.
Second, whenever I see guys mount the rebar to that chinsey little knee bar mount it makes me cringe. The rebar in the A3 dash supports the friggin steering column! And once you chop out the bulkhead going across the car, the rebar is a structural member of the car!
I welded some 1/8" plate steel to the sides of the car, and then welded the plate to the end of the rebar, and bolted it together using 5/16" bolts on each side.
Its not the greatest detail here, and really hard to see, but heres the only pic of what I did...Some guys weld em in which is great for strength, but I wanted to be able to remove it to replace the heater core or A/C evaporator when the time comes...
Birds eye view.