Best bet to make a close match is to purchase another seat with an intact back cover. A good upholstery man (becoming hard to find) should be able to replace the damaged panels with the salvaged material. Saw a nutmeg seat recently on FleaBay that may work for you, or put up a post in the wanted section.
If you aren't concerned about entirely OEM pieces, there are decent reproduction seat covers on eBay as well. Last I remember, several members here were picking up swatches of material to compare to original.
The re-pop material I’ve seen is a different shade of nutmeg compared to a original set I have. The original seats may actually darken some with wear and age. If you find some re-pop, ask them to send you a sample to compare color.
Thanks for all of the feedback. I checked with Austin Jeeps, but they were all out. However, I actually stumbled on someone who has the large back piece of an original Nutmeg seat, and they’re selling it to me for $45 shipped. I’m a happy camper!