Traditional Bottom Bracket Dust Cover Alternatives

[J Imler]

Since Riv started selling the traditional bottom bracket, I've been thinking of installing one on one of my bikes. I have an older Shimano 600 bottom bracket, which seems quite nice. Unfortunately, I'm in need of a mounting cup dust cover for at least one side, as it was broken at some point. See photos for more details.

I've had no luck tracking this plastic bit down online. Any alternatives to consider? An o-ring of equal size, or maybe just don't worry about it?

Thanks