Quite a few that are almost horiz., but not quite.
Galco makes a fairly popular one plus I've seen lots of custom holster makers who offer one, might have to just search if you don't get enough exact suggestion here.
I have one of these and it's actually pretty decent, but you don't want a large gun back there unless you have a very stiff belt, the weight of the ammo in the mag will try to flop it over. 4 Position Handgun holster
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