When I make a keeper, I generally leave it a slight bit loose for ease of buckling. If you get one a little too snug, you can wet mold it and stretch it to fit, assuming you mold it a slight bit loose. For that, I generally cut a piece of hardwood or plastic to put in a ziplock and press into the keeper when it's wet. I don't know if a full half a thickness of leather is necessary but you want the belt to slide easily through it. I generally stitch or rivet keepers into a loop before putting them on a belt.
Here's a photo of what I mean. In the photo I'm wet molding a belt loop, but it's the same process.
Here's a keeper I made too tight so I wet molded it and stretched it to the right size before assembling the belt. Notice how nicely the keeper lies close to the belt.