Found a good crossmatch for rear cargo area seal

In case anyone is looking for a generic alternative to the original part, I can confirm this one is a good match and fits well, with excellent grip on the lip around the opening and plenty of range to pinch and stretch the cargo cover into place:

Vertical Bulb Rubber Seal | Bulb Diameter: 0.51 inch (13mm), Grip Range: 0.039 to 0.12 inch (1-3mm)

Amazon also sells it:

https://www.amazon.com/dp/B0191KFEIW

It's about $1.50/foot depending on the length roll purchased. I'll post some pics below.

Original (left) compared to new (right):
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New installed:
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New installed:
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Looks like the same seal shape/profile is also used around the front spare tire well, however the original front seal on my 328 has some glued/fused joints in the lower corners, so a generic replacement from a bulk roll would need some glue tweaks to be a perfect match (probably not super important).

I also attempted to rescue my old vinyl cargo cover, which was in excellent shape everywhere except the front edge. I think age/heat caused the vinyl to shrink, and that front edge could no longer fully reach the front lip to get pinched into place with the seal. So I trimmed off the old front edge and glued on a strip of black vinyl to extend the edge by a couple inches. It seems to have been successful, is not really noticeable, and I was glad to preserve the original part and save a few hundred dollars on a replacement. But I will reserve final judgement until after seeing how the glue holds up to time/heat over the next couple months. Worst case, I am out $25 and ~1 hr time for the fix attempt.

The fix:
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Done!