Hi James,
The Diamond Pattern RHD Firewall pads are (were) available from 'Authentic Classics' and 'Vintage Euro Parts'. Honestly I cant remember which one was which as I purchased them a while ago.
All I remember is that in my case the one that was delivered on the cardboard roll was easier to fit (more pliable) and didn't delaminate.
As I said, the one that delaminated might have been a quality issue with that batch and was immediately refunded by the vendor.
To Maruo12's point - I don't expect it would have much impact on heat and noise transfer either but I would still select it for aesthetics. When you invest what I have in a total restoration it would be senseless to do otherwise.
On my 1969 280SL I managed to retain and restore the original firewall pad - even though the judges marked me down in a concours entry. I was never going to destroy it to get that extra point. (see below)
Regards,