One thing you can try is to set the x-scale of the prop to -100%. (Or if it's already scaled, set it to negative whatever the scale is. -120%, whatever.)

It will look weird in preview, but usually renders all right. You may have to force normals forward or check remove backfacing polys.

There's also this program from Dimension3d:

https://www.renderosity.com/rr/mod/bcs/?ViewProduct=80640

Content Mirror is a standalone program, so no worries about Python versions. It is Windows-only.

It's really easy to use, and the results are great. I mirror hair props a lot, and I think there was only one that didn't look right when mirrored with Content Mirror. It keeps the morphs and all. It will also mirror clothing.