Snap on sign mount for T-posts.  PETG works well,  as does ASA. The design is with FreeCAD and is setup for 8mm screws, but you can modify it easily. I use these screws from Amazon:

https://www.amazon.com/dp/B08BJYG4GL/ref=cm_sw_em_r_mt_dp_840SR4DAD6T6M3WSAG9P?_encoding=UTF8&psc=1

M8-1.25x8mm 304 SS