Regarding the sensitivity of HEAT fuzes, I'm not sure that the material hit is very relevant at the impact velocities considered.
Think about the "necklace" of the Merkava Mk3/4: there are simple weighted chains dangling from the turret basket, and they are supposed to (and I guess reliably tested to do so) detonate RPG rounds at a standoff from the turret.
I'm out on a limb with my physics here, but I'd say that the inertia of the chains (in this example) prevents them from being significantly displaced before the fuze fires.
The same thing can probably be said of any unarmored vehicle bonnet, given the material is hard enough.
Admittedly, a canvas top has a chance of folding around the fuze probe, or ripping open, letting the fuze tip through, and stop only the thin and conductive warhead cover,making the round a dud. As far as I know that was how RPG fences worked back in Vietnam.
Just my 2c, don't ask me to back that up with calculation!
