whats the price and pics of stiffener you using? i understand there is red color and transparent type, not sure which type is good, and yes i still knock into my exhaust every now and then.
Go for the transparent type as its made of silicon, the red colour is made of rubber.
I have a pair of transparent type installed 4+yrs ago and still in good conditions.
For comparison, you can visit autobac, try squeezing the transparent then the red ones, the transparent ones are much stiffer but they are still no use in preventing knocking into the exhaust while reversing. It just prevent the rear from squatting while fully loaded.
mine was installed at autobac at $80 but again, that was 4yrs ago