I used this idea back in the 90s. I cut up an old T-shirt and repeated it 30 years later.

In the dashing 90s, money bags captivated us with their practicality and convenience. Everything valuable is always on you. There are several compartments and all are closed with a zipper, and this latch lock is so cool.
Since then they have been in use and everyone has them. You can’t do without them when walking and traveling. In addition, a belt bag is a fashion accessory; you can put the most necessary things in it - a passport, ticket, money, bank card, keys, napkins.
For summer, a thin, light handbag is suitable.
Let's make one for ourselves from a T-shirt.
I have a striped T-shirt that I no longer wear, the bag will be made from it, especially since I already have decorations - a design with rhinestones.
I cut out a rectangle measuring 22x31 cm, from which the handbag itself will be sewn.
You will also need a zipper, at least 30 cm long.
Since the knitwear stretches, I pressed 31 cm strips of interlining along the sides.
I stitched one side of the zipper, then put a finishing stitch on the front side, and did the same on the other side, having first unzipped the zipper.
I made folds on the sides and directed them towards each other towards the zipper.
I cut out two 10x10 cm squares from knitwear. I folded them in the middle, with stripes across them.
I stitched and turned it inside out so that I got a cylinder with a fold on one side.
I put this part with an open cut into the bag where the folds were and stitched it.
When I turned it inside out, all the seams were inside.
The strap can be cut from a T-shirt or use a ready-made ribbon.
The strap consists of two parts, its total length must be measured on yourself. It is worth considering that the handbag should fit freely on the figure.
I placed the belt in the inserts and stitched them together. I sewed a fastener onto the opposite edges.
The handbag turned out to be thin, it doesn’t stick out, when you put things inside it stretches.
You have everything with you in one place, no need to stuff things into your pockets and at the same time your hands are free.
Isn't it convenient? And how!
All the best!