You are most welcome and I will be very curious to find out. Thanks!
If worst comes to worst, I will be happy to design a plug for you that RTV's in from the outside with a flange to prevent it from possibly being sucked in.
I could also design it to have small ribs to make a tight press fit.
Because on the V8s the plug is on the top side that shouldn't be an issue, whereas on the V6s it is on the bottom and the tube is very thin so it needs something to clamp it into place (hence the two part inner/outer design).
The closed cell foam and fender washer are available from almost any home improvement store.
I used this foam specifically-
https://www.lowes.com/pd/Frost-King-...lation/3427634
It is also very useful stuff when chasing rattles and air leaks in cars so I always keep a roll of it on hand.
And it can be used for its original purpose (pipe wrap) if your car's A/C pipes sweat a lot or you just want to cover them up.
But any closed-cell foam tape that is about 1/8" thick will work fine.
Wrap it around the plug face against the firewall.
You don't want to go too thick though because it will only make the plug more difficult to push in while twisting it into position.
The fender washer is optional, however if you can find one it does help clamp the plug securely against the firewall and prevent it from warping (it is only printed plastic after all).