Hi Monica,
I'm Mike and I have been around guns, military and law enforcement for awhile and have made many a holster/duty rigs for friends. So here are my .02 cents. First, the person your making it for are they small, medium or large framed? Second what size semi auto and revolver will be carried, barrel length? You mention carrying the gun horizontal, well if the person is small in stature and they want to carry a full size .45 1911 or 1911 long slide (6" barrel) or a revolver with a long barrel it will protrude out the back making it look obvious that they are carrying. Vertical or horizontal, I have done a slight 20-25 degree angle off of vertical which is nice and comfortable for carry and easy to draw the gun. Horizontal will also require a good retention device so it does not fall out, can't have a nice snug friction fit like on a belt holster, as the person goes to draw and the gun is stuck. You may already know that you will need to add buckles so the person can adjust the straps and also add one strap on the bottom of each holster that they can attach to their belt so the holsters do not flop around when walking or if they are a cop, chasing a bad guy. Hope this was helpful.