Once again, the Mythbusters build team is just being goofy.
The clown-car-full-of-balloons thing?
The lab setup is completely non-analogous to the crash scenario.
And, as I watch this, they've discovered that neither easily-popped balloons nor extra-sturdy balloons have the desired effect, so now they're back in the lab trying to get the lowest possible peak accelerometer reading when they release a weighted plunger atop a cylinder of balloons.
Er, no. Not unless you suddenly have the floor rise two feet.
In the setup they've got? Lowest peak acceleration would be obtained with a solid cylinder of concrete. The plunger wouldn't move at all, giving a peak reading of 1G exactly.
This would not work well if the floor suddenly rose two feet... or if, analogously, the whole system were moving rapidly and then came to a stop within two feet of travel.
What you're looking for is something intermediate, which will ease the deceleration such that the clown (a) doesn't just go right through and crash into the dashboard, but (b) also isn't so firmly restrained as to experience the full deceleration of the passenger compartment (which, admittedly, ought to be substantially less than that experienced by the front bumper).