Hollander shows all '68 - '72 two and four doors taking the same rear window- #3090. Unlike what happened with the Chevelle, where two and four door cars had different rear glass. So perhaps a panel from a four door would work.
When Mark Reeves of "Then and Now" restorations was working with, I believe, AMD to make the 70-72 repro parts .. he did the deck filler and brought it and all the other repro parts to the BPG event for their "unveiling". There, he asked if he could take dimensions off my 69 GS convert. to see if the deck filler panel would work for 68/69 .. .. his conclusion was that with minimal work (if any at all), it should work well I think The Parts Place is now manufacturing and selling these parts. DISCLAIMER - I have NO FACTUAL KNOWLEDGE IF IT WILL WORK
One of them is on the 69 Motion car. I think Nick had it installed in less then 1 hour and that included cutting the old one out. If you use a new panel you will also need to put the studs back on for the window chrome. Duane