Quoted from beelzeboob:

Yes, that's what you need. Just take out the entire back plate and line the sides and back with the dead drop foam. It'll take you one or two pieces, but order five or so...you'll use it for other applications. And no, the stuff at Home Depot is for air conditioning...not for deadening the velocity of a pinball.
And you can try to leave the back plate in, but Chris at HEP and I both tried different things and the only one that truly worked was to just remove it altogether.

Quoted from beelzeboob:

Yes. Trial and error. Literally. You might also have to bend the small metal piece that guides the ball onto the POA. You want to make sure that it allows you to dump the ball into either lane. If not adjusted just right, you'll never get it into the left lane.
But really, it will take quite a bit of tweaking and patience.

Quoted from beelzeboob:

Ah! Well, that's pretty normal. Mine is leveled correctly, and I usually have to alternate the ball from one side to the other to keep it from flying off the wall and into the hole. I think it may have been designed that way with those little scoops on the side that fling the ball into the center.
What you can consider doing is putting a washer or two under the front support of the POA. This will lift it a bit so the ball moves a little more slowly and doesn't fly off the wall. But that would be cheating.

Quoted from Hemispheres:

Hands down the absolute best fix is to remove the deflector and line the back to the left with one continuous piece of dead foam ending 34 of the way up at the the dish. Then we install a protector (we just make one with 1/8") and this will protect the opto and install longer screws in the plastic mounting screws. The ONLY way it is not staying in is if it rattles at the front off the posts and is not a direct shot. if it's direct no matter how hard it is 100%. Have done dozens this way. You MUST remove the deflector if you want it 100%. We also add a cliffy and then the shallowest screws to hold it in. This was a test protector. We make them from clear for better appearance.

Quoted from Hemispheres:

Hands down the absolute best fix is to remove the deflector and line the back to the left with one continuous piece of dead foam ending 34 of the way up at the the dish. Then we install a protector (we just make one with 1/8") and this will protect the opto and install longer screws in the plastic mounting screws. The ONLY way it is not staying in is if it rattles at the front off the posts and is not a direct shot. if it's direct no matter how hard it is 100%. Have done dozens this way. You MUST remove the deflector if you want it 100%. We also add a cliffy and then the shallowest screws to hold it in. This was a test protector. We make them from clear for better appearance.

Quoted from KornFreak28:

What did you use to protect the opto?

Quoted from beelzeboob:

Haven't you heard? That's cheating! I'm glad somebody agrees with me around here...

I'll answer that: Nothing. The opto is a ramp entrance opto and is on the other side of the small guide rail. It's nearly impossible for the ball to hit that when being ejected--it would hit the rail first and careen away from the opto. Bigger concern is the diode on the underside of the upper playfield direcly above it, which normally gets squashed. But some enterprising young German (Austrian?) made a shield for it that's somewhere in this thread, I believe. Dude should sell that, stat!