Over the years, I've tried various unsuccessful ways of making the "gap in the back" smaller. Belts to keep one's pants close to the body are a hassle to me. Any sewing alterations failed. And then I had a success.
On a fairly old pair of jeans, I tried sewing small darts/wedges on each side of the back waist. This made the fit so, so much better. The stitching even held up when I pulled the pants on and off. This was always my problem before - the stitching would break. I think it was the placement of the wedges before.
To start this successful alteration, I safety pinned the extra fabric out of the pants on each side. Then I measured and evened out how much to take out of each side so that the measurements were the same. Then I sewed the dart/wedge. There is a tiny pucker where I ended the stitching, but my shirt usually covers this. There is a tiny bit of gap, but that is needed for ease and to get the pants over my hips. What is thankfully missing is the kangaroo pocket created by my spine and back waistband.
Now these jeans have gone from purely functional to a pleasure to wear. This alteration worked so well, that I did it on a pair of casual twill work trousers. From measuring to snipping off the last threads, the entire process takes less than half an hour, and that includes threading the machine.
This may not be the official way to alter a waistband, but it works for my purposes.