What are all you carrying your spare magazines in? In a double stack 9mm carrier the mag is loose and sits too low in the holster to get a good grip on it. I've tried a 1911 mag carrier but it's too tight.
Other than ordering a custom made mag carrier, what's out there?

I got an adjustable plastic one from Alien Gear.

Magholder, LLC offers a horizontal magazine carrier option, with the caveat that it is not leather.

Magholders are, from my experience with them for both 1911 and Glock magazines, very useful and effective.

The owner also happens to be a member here on SF.

Here’s a link to their site, specifically to their SIG section for your mags:

https://magholder.com/products...riant=36796695773347

Look at Desantis. I have a couple leather pouches that work for both my P365 and my Hellcat.

If you want decent leather P365 mag holders that will ship quickly, grab a couple $20 ones from Don Hume. P365's fit code is 100B.
https://www.donhume.com/index....th=9&products_id=766

Or, if you're willing to consider kydex, I highly recommend grabbing some P365-specific kydex mag holders from Kytex Gear. $19 apiece. Usually ships in a week or so.
https://kytexgear.com/product/...88291168212890625000

The leather, single mag carrier SIG sells, made by Mitch Rosen is perfect for OWB belt carry.