In reply to Josslad:
I had a Montane 10L one that’s quite customisable size wise. They are S/M and M/L and comfortable enough to size up/down. I am not big fan of bottles being so low and the bottle pockets not being the same. It’s just too much hassle with the pipes running up. I eventually lost too much weight and it’s too big for me now.
I have Salomon Slab Sense 5 and Salomon Adv Skin 12 now. The former is great for races, but it’s a bit thin and doesn’t fill me up with confidence I’m not gonna lose stuff. Adv Skin 12 is a bit thicker material, but contains zips and has more structure to it. It’s pretty stable when empty.
I still have Slab Sense 5 in Medium that’s just too roomy for me. Small is perfect, until you gain few kilos again. It’s a bit like a shirt, you can’t fluctuate too much in size.
Have you seen Salomon Agile 2? It’s cheap, simple and would be large enough for your needs. It rubs into my sides/armpits on longer runs (18-20 miles), but would be my choice for training.