Firewall Insulation Pad (best plugs to retain???)

[BamaNomad]

I've been TRYING to install a firewall insulation pad in my '57 Nomad. The clips/plugs that came with the REM pad are made of *plastic* (hard plastic) and are made *similar* to the originals, BUT I've been unable to get them thru the holes in my firewall. I've installed original type plugs (long ago - 30+ yrs ago) without issue, but apparently these are no longer available and the REM repros are *essentially unusable*.

There are some 'rubber' plugs available for later year GM products (not sure if hole size is the same, but I suspect? it is?), but these plugs are also made differently and are different in appearance where they protrude thru the firewall. Do they work? or do they leave the pad 'loose'?
Originals were made like this, but worked better. This one is stated to be for a '62 Nova, but not sure if the hole sizes are compatible? or more pliable than the REM pieces I have...?

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These are stated to be made from 'rubber' and more pliable, and are stated to be for 'GM products from '55 to 81', but they may not hold the pad tightly to the firewall, and are obviously made different ...
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Has anyone found a solution to this problem? Or have any comments about the above parts?