The portable pickleball net delivered from Amazon was missing this end cap. This is a quick print to replace it. Printed in TPU. The cap fits over the vertical posts and has a lip to retain the net.

Pickleball Net:
https://www.amazon.com/dp/B08L67JSC3?th=1&linkCode=ll1&tag=bschiav-20&linkId=7c616dc6f340bc1a47d23af795434edd&language=en_US&ref_=as_li_ss_tl

I assume this would work for brands, seems this is a standard design.