Thank you, crorris!  Your top link's product worked: https://www.amazon.com/dp/B07WNTFMCB?ref=ppx_yo2ov_dt_b_fed_asin_title It's a little loose but so far hasn't popped out while riding.  I used a cable tie to keep it handy per the photo.

Thanks, but it's the little black round rubber one that seals where the charging cable attaches.  It's attached to the battery via a rubber flap. 

This is the small black rubber plug that you insert into the charging port after you disconnect your charger on a Radrunner Plus (I assume most/all Radpower bikes are the same).  Mine split in two.  Now if my e-bike is ever in the rain, the battery may short. 

I just spent about twenty minutes looking for one of these on line but can't find one anywhere. 

Any ideas?