I am looking for a holster to carry a mag horizontal on my belt. Specifically it will be for an FN 509 24 round mag.
I prefer something universal, where I can use it for single stack and double stack options as well. Not sure if such a product exists.

Not sure if it's already on the market, but SBF Custom Kydex could probably make it for you

They definitely have variety.. HAHA

I took a mag holster for an M&P shield, heated it up, jammed in FN 509 mag  and reformed it. Changed clip orientation to horizontal. Fits perfectly.  It works but because the clip is so far to one side, it kicks out the heel of the mag a lot.  I am considering heating it back up and pushing the mag all the way through the kydex to a more center position.. But I am worried that it will ruin retention. I also don't like the bullets being unprotected and the chance of a round falling out somewhere.

Maybe I should buy some kydex and roll my own. Seems easy enough.

17 round mags are perfect in this setup. Maybe I should just carry a 17rounder for a backup mag?  I know Ill probably never need it, but having 24+ 17 + 1 rounds on me daily is sort of comforting.  

Haven't tried but ordering Stealth Gear SG-180

That may actually be the ticket!