Get a pair of suspenders, get a strip of nylon/elastic, use a shell to measure proper size, sew the nylon/elastic onto the suspenders.
Probably the cheapest "chest rig" for holding easily accessible shells. Well, even cheaper would be to wear two shirts, tuck them in and tighten your belt, then dump shells between the two shirts for the cheapest dump pouch ever.
The idea works with just about any suitable "backing" but the main downside is that if you use a soft/stretchy backing like regular cloth, as soon as you put weight on the webbing/molle/loops or whatever you attach it just sags and flops around. My first chest rig attempt failed miserably due to that. Backing was denim, anything heavier than a pen and paperclip and it just sagged and flopped. I had to keep adding support straps until it was more bondage outfit than chest rig.
Unless you have access to some good stiff backing material, it's probably easier and more cost effective to buy some surplus stuff.